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Electrochemical intercalation of sulphuric acid into graphite in the presence of molybdenum trioxide
Solid State Ionics 157 (2003) 51 – 55 www.elsevier.com/locate/ssi
Electrochemical intercalation of sulphuric acid into graphite in the presence of molybdenum trioxide J.M. Skowron´ski * Institute of Chemistry and Technical Electrochemistry, Poznan´ University of Technology, ul. Piotrowo 3, 60-965 Poznan´, Poland Received 25 May 2001; received in revised form 21 January 2002; accepted 31 January 2002
Abstract Electrochemical intercalation – deintercalation of 18 M H2SO4 into graphite is examined in the acid solution free and admixed with 0.5 M MoO3. The differences observed in the kinetics of reactions resulting from the presence of MoO3 in H2SO4 allow an assumption that negatively charged molybdenum oxide and/or its intermediates are intercalated into graphite galleries together with sulphuric acid producing ternary H2SO4 – MoOx – graphite intercalation compound (GIC). D 2002 Elsevier Science B.V. All rights reserved. Keywords: Graphite intercalation compounds; H2SO4; MoO3; Cyclic voltammetry
1. Introduction Graphite intercalation compounds with sulphuric acid (H2SO4 – GICs) belong to the class of acceptor compounds because during the process of intercalation, the charge transfer from the k band of graphite to the intercalate occurs. Most intercalates may enter the graphite lattice provided some strong chemical oxidizer extracts electrons from the graphene layers. Instead of strong oxidizers added to the reaction solution, the process of intercalation can be forced electrochemically. As anodic oxidation of the graphite host occurs, the negatively charged species solvated with H2SO4 molecules (HSO 4 2.5 H2SO4) enter into the interlayer spacings of graphite [1 –3]. Most of GICs comprise only two components, graphite and one intercalate. Recently, an increasing number of
works is attempted to prepare GICs in which two or more intercalates are accommodated in the graphite lattice (ternary GICs, quaternary GICs, etc.). Graphite bi-intercalation compound (GBC) is a graphite system in which alternating layers of two or more different intercalates occupy separate interlayer spacings of graphite and the intercalate layers are separated by one or more graphene layers [1], e.g., CrO3 – H2SO4 – GBC, CrO 3 – HClO 4 – GBC [4], ZnCl 2 – CrO 3 – H2SO4 – GBC [5]. In the present contribution, the electrochemical intercalation of sulphuric acid into graphite is carried out in 18 M H2SO4 containing molybdenium trioxide and the influence of MoO3 on the kinetics of the intercalation –deintercalation process is examined.
2. Experimental *
Fax: +48-61-665-2571. E-mail address: [email protected] (J.M. Skowron´ski).
The intercalation – deintercalation process was carried out in 18 M H2SO4 free and admixed with 0.5 M
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 1 8 9 - 3
52
J.M. Skowron´ski / Solid State Ionics 157 (2003) 51–55
MoO3 using an H-type cell in which the working and the counter electrode compartments were separated by a glass frit. The working electrode, in the form of a graphite flake (99.9 wt.% C) bed, was placed on a platinum screen. The counter electrode was a platinum spiral, whereas the reference electrode was Hg/ Hg2SO4/1 M H2SO4 (0.674 V vs. SHE) connected to the solution under investigation by a Luggin capillary. Potentials measured against this electrode are designated E in Figs. 1– 4. Slow cyclic voltammetry (CV) method was used with a scan rate 4 AV s1 (PAR 273A potentiostat/galvanostat, EG&G). For comparison, voltammograms for a platinum electrode (2 cm2) were recorded in 18 M H2SO4/0.5 M MoO3 solution. Two modes of voltammetric runs were used. In one series of runs, after starting the measurement at the rest potential of electrode, the potential was changed in the positive direction until the maximum potential 1.1 V was reached. Then the direction of polarization was automatically reversed and the potential was reduced down to 0.05 V (Figs. 1– 3). In the other series of measurements, the potential was first decreased down to 0.05 V and after the polarization reversion, the potential was increased up to the vertex potential 0.5 V (Fig. 4). All measurements were performed at a temperature of 20 jC.
3. Results and discussion CV curves recorded for a graphite flake bed electrode in pure 18 M H2SO4 are depicted in Fig. 1. The multiple peak a recorded in the potential range up to 0.5 V during the forward potential sweep is associated with the formation of higher stages of H2SO4 – GIC (nz2). A stage number n determines the number of graphene layers between which adjacent intercalate layers are sandwiched (n=l is assigned to pure graphite). Within the peak at 0.43 V stage-2 H2SO4 – GIC is formed. Peak b, starting at 0.7 V, corresponds to the formation of stage-1 H2SO4 – GIC from stage-2 H2SO4 –GIC [2,3]. Within the plateau c the ionization of the stage-1 compound occurs and the corresponding reduction takes place within the plateau cV [6]. All the peaks recorded during the reverse potential sweep are associated with the decomposition of the following stages, e.g., peak bVrelates to the conversion from stage-1 to stage-2 H2SO4 – GIC.
Fig. 1. Cyclic voltammograms recorded for a graphite flake bed electrode in 18 M H2SO4. Potential range: 0.05 X 1.10 V. Scan rate: 4 AV s1. Cycle 1 (—), cycle 2 (- -).
The presence of MoO3 in 18 M H2SO4 brings about considerable changes in the process of intercalation (Fig. 2). The rest potential of the electrode was 0.23 V shifted to the positive potentials and a relatively large peak appeared before the anodic peak corresponding to the formation of stage-2 H2SO4 – GIC was revealed. The shape of the main peak at 0.7 V (B) remained unchanged but the charge of this peak increased as compared to peak b (Table 1). After the reversal of polarization, the double cathodic peak BV was observed instead of the single cathodic peak bV appearing in pure H2SO4. This difference allows the assumption that in the course of the anodic run HSO 4 ions are intercalated between the interlayer spacings of graphite together with ionized MoO3 species. Such an explanation is supported by the fact that no cathodic reactions take place in the same potential range at the surface of a platinum electrode immersed in H2SO4/MoO3 solution (Fig. 3). As can be seen from this figure, the cathodic reduction of dissolved MoO3 begins to occur at the potential about 0.45 V and expands down to the potential 0.05 V with the minimum attained at about 0.17 V. In this range of potential, the solution becomes blue in colour which indicates that the mixed system composed of tetraand pentavalent molybdenum compounds is formed (so-called molybdenum blue) [7]. The cathodic peak appearing again at about 0.24 V after the reversal of polarization can be related to unblocking the electrode surface covered with the products of MoO3 reduction. The broad anodic peak, with the maximum at 0.6 V,
J.M. Skowron´ski / Solid State Ionics 157 (2003) 51–55
Fig. 2. Cyclic voltammograms recorded for a graphite flake bed electrode in 18 M H2SO4 containing 0.5 M MoO3. Potential range: 0.05 X 1.10 V. Scan rate: 4 AV s1. Cycle 1 (—), cycle 2 (- -).
recorded for platinum electrode in the second cycle is associated with the oxidation reaction resulting in complex compounds having a valency between +5 and +6 (e.g., MoOd 3 , HMoO4 ) [7]. As shown in Table 1, the cathodic charge, Q2d, calculated for the reaction of graphite in H2SO4/MoO3 solution in the potential range corresponding to the conversion from stage-2 to stage-l GIC, is 2.4 times higher than that obtained in pure H2SO4, whereas the ratio of anodic charges, Q2i, calculated for the reaction proceeding in mixed (265.2 C g1) and in pure
53
electrolyte (133.7 C g1) equals 1.98. The higher charges observed for the reaction occurring at the graphite electrode in the presence of MoO3 and blue colour of the solution at the graphite/electrolyte interface indicate that molybdenum ions of valency lower than six are formed like for the platinum electrode. However, in contrast to the irreversibility of the cathodic/anodic reactions proceeding on the platinum electrode, the reaction on the graphite electrode exhibits a good reversibility ( Q2d/Q2i=1.08, where Q2d and Q2i denote the deintercalation and intercalation charges in the potential range AVand A, respectively). A number a little higher than 1 is likely caused by the fact that the deintercalation process involving the conversion of the stage-1 to the stage-2 compound does not occur completely at the potential range BV(the reversibility degree Q1d/Q1i=0.76). The improved reversibility of reactions cannot be attributed only to difference in chemical nature of platinum and graphite because there are important findings on CV curves suggesting that the reason for the high reversibility may be associated with intercalation of molybdenum anions in the graphite lattice. Contrary to the anodic peak recorded for the platinum electrode in the potential range 0.5 –1.1 V (Fig. 3), for the graphite electrode no anodic peak is observed in the same potential range (Fig. 2) and the main anodic peak at 0.7 V is not even deformed. Despite this, the
Table 1 Electrochemical data calculated based on voltammetric curves recorded during the second intercalation/deintercalation cycle 18 M H2SO4 Intercalation Intercalation potential, Ei [V] Potential of stage (2!1) transformation peak, E1i [V] Potential of stage (1!2) transformation peak, E1d [V] Intercalation charge for stages (l!2), Q2i [C g1] Intercalation charge for stages (2!1), Q1i [C g1] Total intercalation charge, Qti [C g1] Total anodic charge, Qta [C g1] Deintercalation charge for stages (1!2), Q1d [C g1] Deintercalation charge for stages (2!l), Q2d [C g1] Total deintercalation charge, Qtd [C g1] Total cathodic charge, Qtc [C g1] Q1d/Q1i Q2d/Q2i Qtd/Qti Qtc/Qta
0.192 0.722
18 M H2SO4+0.5 M MoO3 Deintercalation – –
– 133.7 120.2 253.9 294.3 – – – –
0.618 – – – – 107.3 119.6 226.9 257.2 0.89 0.89 0.89 0.87
Intercalation 0.321 0.749
Deintercalation – –
– 265.2 132.8 398.0 442.3 – – – –
0.599; 0.551 – – – – 101.0 287.2 388.2 421.0 0.76 1.08 0.97 0.95
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J.M. Skowron´ski / Solid State Ionics 157 (2003) 51–55
Fig. 3. Cyclic voltammograms recorded for a platinum electrode (1 cm2) in 18 M H2SO4 containing 0.5 M MoO3. Potential range: 0.05 X 1.10 V. Scan rate: 4 AV s1. Cycle 1 (—), cycle 2 (- -).
broad and double cathodic peak is observed at about 0.6 V after the polarization direction is reversed. Such a behaviour can be understood on assumption that in the course of the first positive sweep bisulphate ions are intercalated in the graphite galleries together with negatively charged molybdenum ions forming ternary d (HSO 4 /MoO3 ) – GIC. It is worthwhile to recall that similar splitting of the main deintercalation peak is a characteristic feature of the cathodic reduction of ZnCl2 –H2SO4 – GBC [5]. Moreover, the results of the in-situ XRD and CV measurements have shown previously [8] that the domains of H2SO4 – CrO3 – GBC are present in the graphite lattice at the potential 0.8 V which means that they are already created before the stage-1 compound is formed. A detailed consideration of CV curves recorded in the potential range limited to the formation of stage-2 GIC (Fig. 4) provides new evidence for the formation of GIC with the participation of MoO3 compounds. As mentioned above, the CV curves presented in Fig. 4 were recorded in such a way that the process started in the direction of negative potentials. Owing to this, it was possible to determine whether or not some cathodic reduction occurs immediately after starting the measurement. For the process examined in pure H2SO4 (Fig. 4, curve 1), no current was recorded from the rest potential (0.06 V) to the vertex potential 0.05 V and then until the potential 0.21 V was reached. The completed curve is composed of a mirror set of intercalation – deintercalation peaks. The left cathodic peak corresponds to the decomposition of dilute H2SO4 –GIC (high-
stages GIC) to pristine graphite. On the voltammogram obtained in the electrolyte containing the MoO3 additive (Fig. 4, curve 2), the cathodic peaks appear instantly after the measurement is initiated. The peaks well match those recorded during the second cycle in the electrolyte free of MoO3. This result proves without any doubt that before starting the potential sweep, higher stages of GIC are chemically formed due to the oxidation of graphite by molybdenum compounds present in the electrolyte. The lateral anodic peaks on curve 2 are quite similar in charge and position to those noted without MoO3 additive (curve 1), which means that electrochemical formation of the corresponding stages of GIC occurs in the same manner. In the centre of the voltammogram, there is the large and broad peak. It can be pointed out with ease that the oxidation reactions involving molybdenum compounds contribute to this peak. However, due to high amplitude of this peak, it is difficult to distinguish between the reactions occurring in the solution and in the graphite lattice (the peaks corresponding to the latter reactions are hidden under the broad peak arising from the former ones). The reversibility of the deintercalation – intercalation process in the shortened potential range is, however, very satisfactory and the total charge is over twice larger than that for the process taking place in pure H2SO4. From comparison of voltammograms recorded in the whole and shortened range of potentials (Figs. 2 and 4), it is clear that the mechanism of cathodic reactions occurring at poten-
Fig. 4. Cyclic voltammograms recorded for a graphite flake bed electrode in (1) 18 M H2SO4 (- -) and (2) 18 M H2SO4 containing 0.5 M MoO3 (—). Potential range: 0.05 X 0.50 V. Scan rate: 4 AV s1.
J.M. Skowron´ski / Solid State Ionics 157 (2003) 51–55
tials lower than 0.5 V undergoes modification if the precedent anodic polarization up to 1.1 V followed by the reduction down to 0.5 V is carried out. Because no features arising from the presence of molybdenum compounds in the electrolyte were observed on the anodic side of voltammogram between 0.5 and 1.1 V, hence the broadening of muliple cathodic peak AV, precedent by the double cathodic peak BV(0.6 V), cannot only be accounted for by the reduction of molybdenum compounds at the graphite/solution interface but results also from deintercalation processes involving the phase transition of intercalation domains encompassing HSO 4 and MoOd 3 intercalates. At this stage of investigation it is not possible to determine the chemical composition of molybdenum compounds taking part in the intercalation process. It is, however, likely that the interaction of MoOd 3 with HSO intercalate brings about the complex ions 4 which are capable of intercalating. Schulz et al. [9] have reported that the chemical process of intercalation in MoO3/CCl3COOH solution can successively occur provided the active system MoO3/CCl3COCl is formed.
4. Conclusions The intercalation process occurring in H2SO4/ MoO3 solution is interesting as a new reversible sys-
55
tem for chemical power sources. As compared to the process observed in pure acid, the discharge (deintercalation)/charge (intercalation) reversibility is as high, whereas the specific discharge capacity is over 1.5 times higher in the presence of MoO3.
References [1] J.M. Skowron´ski, in: H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, Graphite Intercalation Compounds, vol. 1, Wiley, Chichester, 1997, pp. 621 – 686. [2] J.M. Skowron´ski, Synth. Met. 95 (1998) 135. [3] J.M. Skowron´ski, J. Urbaniak, Mol. Cryst. Liq. Cryst. 340 (2000) 277. [4] J.O. Besenhard, E. Wudy, H. Mo¨hwald, J.J. Nickl, W. Biberacher, W. Foag, Synth. Met. 7 (1986) 185. [5] A. Metrot, H. Fuzelier, Carbon 22 (1984) 131. [6] H. Shioyama, R. Fuji, Carbon 25 (1987) 771. [7] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon, Oxford, 1966, pp. 272 – 279. [8] J.M. Skowron´ski, H. Shioyama, Carbon 33 (1995) 1473. [9] F. Schulz, P. Behrens, W. Metz, Synth. Met. 34 (1989) 145.
Electrochemical intercalation of sulphuric acid into graphite in the presence of molybdenum trioxide J.M. Skowron´ski * Institute of Chemistry and Technical Electrochemistry, Poznan´ University of Technology, ul. Piotrowo 3, 60-965 Poznan´, Poland Received 25 May 2001; received in revised form 21 January 2002; accepted 31 January 2002
Abstract Electrochemical intercalation – deintercalation of 18 M H2SO4 into graphite is examined in the acid solution free and admixed with 0.5 M MoO3. The differences observed in the kinetics of reactions resulting from the presence of MoO3 in H2SO4 allow an assumption that negatively charged molybdenum oxide and/or its intermediates are intercalated into graphite galleries together with sulphuric acid producing ternary H2SO4 – MoOx – graphite intercalation compound (GIC). D 2002 Elsevier Science B.V. All rights reserved. Keywords: Graphite intercalation compounds; H2SO4; MoO3; Cyclic voltammetry
1. Introduction Graphite intercalation compounds with sulphuric acid (H2SO4 – GICs) belong to the class of acceptor compounds because during the process of intercalation, the charge transfer from the k band of graphite to the intercalate occurs. Most intercalates may enter the graphite lattice provided some strong chemical oxidizer extracts electrons from the graphene layers. Instead of strong oxidizers added to the reaction solution, the process of intercalation can be forced electrochemically. As anodic oxidation of the graphite host occurs, the negatively charged species solvated with H2SO4 molecules (HSO 4 2.5 H2SO4) enter into the interlayer spacings of graphite [1 –3]. Most of GICs comprise only two components, graphite and one intercalate. Recently, an increasing number of
works is attempted to prepare GICs in which two or more intercalates are accommodated in the graphite lattice (ternary GICs, quaternary GICs, etc.). Graphite bi-intercalation compound (GBC) is a graphite system in which alternating layers of two or more different intercalates occupy separate interlayer spacings of graphite and the intercalate layers are separated by one or more graphene layers [1], e.g., CrO3 – H2SO4 – GBC, CrO 3 – HClO 4 – GBC [4], ZnCl 2 – CrO 3 – H2SO4 – GBC [5]. In the present contribution, the electrochemical intercalation of sulphuric acid into graphite is carried out in 18 M H2SO4 containing molybdenium trioxide and the influence of MoO3 on the kinetics of the intercalation –deintercalation process is examined.
2. Experimental *
Fax: +48-61-665-2571. E-mail address: [email protected] (J.M. Skowron´ski).
The intercalation – deintercalation process was carried out in 18 M H2SO4 free and admixed with 0.5 M
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 1 8 9 - 3
52
J.M. Skowron´ski / Solid State Ionics 157 (2003) 51–55
MoO3 using an H-type cell in which the working and the counter electrode compartments were separated by a glass frit. The working electrode, in the form of a graphite flake (99.9 wt.% C) bed, was placed on a platinum screen. The counter electrode was a platinum spiral, whereas the reference electrode was Hg/ Hg2SO4/1 M H2SO4 (0.674 V vs. SHE) connected to the solution under investigation by a Luggin capillary. Potentials measured against this electrode are designated E in Figs. 1– 4. Slow cyclic voltammetry (CV) method was used with a scan rate 4 AV s1 (PAR 273A potentiostat/galvanostat, EG&G). For comparison, voltammograms for a platinum electrode (2 cm2) were recorded in 18 M H2SO4/0.5 M MoO3 solution. Two modes of voltammetric runs were used. In one series of runs, after starting the measurement at the rest potential of electrode, the potential was changed in the positive direction until the maximum potential 1.1 V was reached. Then the direction of polarization was automatically reversed and the potential was reduced down to 0.05 V (Figs. 1– 3). In the other series of measurements, the potential was first decreased down to 0.05 V and after the polarization reversion, the potential was increased up to the vertex potential 0.5 V (Fig. 4). All measurements were performed at a temperature of 20 jC.
3. Results and discussion CV curves recorded for a graphite flake bed electrode in pure 18 M H2SO4 are depicted in Fig. 1. The multiple peak a recorded in the potential range up to 0.5 V during the forward potential sweep is associated with the formation of higher stages of H2SO4 – GIC (nz2). A stage number n determines the number of graphene layers between which adjacent intercalate layers are sandwiched (n=l is assigned to pure graphite). Within the peak at 0.43 V stage-2 H2SO4 – GIC is formed. Peak b, starting at 0.7 V, corresponds to the formation of stage-1 H2SO4 – GIC from stage-2 H2SO4 –GIC [2,3]. Within the plateau c the ionization of the stage-1 compound occurs and the corresponding reduction takes place within the plateau cV [6]. All the peaks recorded during the reverse potential sweep are associated with the decomposition of the following stages, e.g., peak bVrelates to the conversion from stage-1 to stage-2 H2SO4 – GIC.
Fig. 1. Cyclic voltammograms recorded for a graphite flake bed electrode in 18 M H2SO4. Potential range: 0.05 X 1.10 V. Scan rate: 4 AV s1. Cycle 1 (—), cycle 2 (- -).
The presence of MoO3 in 18 M H2SO4 brings about considerable changes in the process of intercalation (Fig. 2). The rest potential of the electrode was 0.23 V shifted to the positive potentials and a relatively large peak appeared before the anodic peak corresponding to the formation of stage-2 H2SO4 – GIC was revealed. The shape of the main peak at 0.7 V (B) remained unchanged but the charge of this peak increased as compared to peak b (Table 1). After the reversal of polarization, the double cathodic peak BV was observed instead of the single cathodic peak bV appearing in pure H2SO4. This difference allows the assumption that in the course of the anodic run HSO 4 ions are intercalated between the interlayer spacings of graphite together with ionized MoO3 species. Such an explanation is supported by the fact that no cathodic reactions take place in the same potential range at the surface of a platinum electrode immersed in H2SO4/MoO3 solution (Fig. 3). As can be seen from this figure, the cathodic reduction of dissolved MoO3 begins to occur at the potential about 0.45 V and expands down to the potential 0.05 V with the minimum attained at about 0.17 V. In this range of potential, the solution becomes blue in colour which indicates that the mixed system composed of tetraand pentavalent molybdenum compounds is formed (so-called molybdenum blue) [7]. The cathodic peak appearing again at about 0.24 V after the reversal of polarization can be related to unblocking the electrode surface covered with the products of MoO3 reduction. The broad anodic peak, with the maximum at 0.6 V,
J.M. Skowron´ski / Solid State Ionics 157 (2003) 51–55
Fig. 2. Cyclic voltammograms recorded for a graphite flake bed electrode in 18 M H2SO4 containing 0.5 M MoO3. Potential range: 0.05 X 1.10 V. Scan rate: 4 AV s1. Cycle 1 (—), cycle 2 (- -).
recorded for platinum electrode in the second cycle is associated with the oxidation reaction resulting in complex compounds having a valency between +5 and +6 (e.g., MoOd 3 , HMoO4 ) [7]. As shown in Table 1, the cathodic charge, Q2d, calculated for the reaction of graphite in H2SO4/MoO3 solution in the potential range corresponding to the conversion from stage-2 to stage-l GIC, is 2.4 times higher than that obtained in pure H2SO4, whereas the ratio of anodic charges, Q2i, calculated for the reaction proceeding in mixed (265.2 C g1) and in pure
53
electrolyte (133.7 C g1) equals 1.98. The higher charges observed for the reaction occurring at the graphite electrode in the presence of MoO3 and blue colour of the solution at the graphite/electrolyte interface indicate that molybdenum ions of valency lower than six are formed like for the platinum electrode. However, in contrast to the irreversibility of the cathodic/anodic reactions proceeding on the platinum electrode, the reaction on the graphite electrode exhibits a good reversibility ( Q2d/Q2i=1.08, where Q2d and Q2i denote the deintercalation and intercalation charges in the potential range AVand A, respectively). A number a little higher than 1 is likely caused by the fact that the deintercalation process involving the conversion of the stage-1 to the stage-2 compound does not occur completely at the potential range BV(the reversibility degree Q1d/Q1i=0.76). The improved reversibility of reactions cannot be attributed only to difference in chemical nature of platinum and graphite because there are important findings on CV curves suggesting that the reason for the high reversibility may be associated with intercalation of molybdenum anions in the graphite lattice. Contrary to the anodic peak recorded for the platinum electrode in the potential range 0.5 –1.1 V (Fig. 3), for the graphite electrode no anodic peak is observed in the same potential range (Fig. 2) and the main anodic peak at 0.7 V is not even deformed. Despite this, the
Table 1 Electrochemical data calculated based on voltammetric curves recorded during the second intercalation/deintercalation cycle 18 M H2SO4 Intercalation Intercalation potential, Ei [V] Potential of stage (2!1) transformation peak, E1i [V] Potential of stage (1!2) transformation peak, E1d [V] Intercalation charge for stages (l!2), Q2i [C g1] Intercalation charge for stages (2!1), Q1i [C g1] Total intercalation charge, Qti [C g1] Total anodic charge, Qta [C g1] Deintercalation charge for stages (1!2), Q1d [C g1] Deintercalation charge for stages (2!l), Q2d [C g1] Total deintercalation charge, Qtd [C g1] Total cathodic charge, Qtc [C g1] Q1d/Q1i Q2d/Q2i Qtd/Qti Qtc/Qta
0.192 0.722
18 M H2SO4+0.5 M MoO3 Deintercalation – –
– 133.7 120.2 253.9 294.3 – – – –
0.618 – – – – 107.3 119.6 226.9 257.2 0.89 0.89 0.89 0.87
Intercalation 0.321 0.749
Deintercalation – –
– 265.2 132.8 398.0 442.3 – – – –
0.599; 0.551 – – – – 101.0 287.2 388.2 421.0 0.76 1.08 0.97 0.95
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J.M. Skowron´ski / Solid State Ionics 157 (2003) 51–55
Fig. 3. Cyclic voltammograms recorded for a platinum electrode (1 cm2) in 18 M H2SO4 containing 0.5 M MoO3. Potential range: 0.05 X 1.10 V. Scan rate: 4 AV s1. Cycle 1 (—), cycle 2 (- -).
broad and double cathodic peak is observed at about 0.6 V after the polarization direction is reversed. Such a behaviour can be understood on assumption that in the course of the first positive sweep bisulphate ions are intercalated in the graphite galleries together with negatively charged molybdenum ions forming ternary d (HSO 4 /MoO3 ) – GIC. It is worthwhile to recall that similar splitting of the main deintercalation peak is a characteristic feature of the cathodic reduction of ZnCl2 –H2SO4 – GBC [5]. Moreover, the results of the in-situ XRD and CV measurements have shown previously [8] that the domains of H2SO4 – CrO3 – GBC are present in the graphite lattice at the potential 0.8 V which means that they are already created before the stage-1 compound is formed. A detailed consideration of CV curves recorded in the potential range limited to the formation of stage-2 GIC (Fig. 4) provides new evidence for the formation of GIC with the participation of MoO3 compounds. As mentioned above, the CV curves presented in Fig. 4 were recorded in such a way that the process started in the direction of negative potentials. Owing to this, it was possible to determine whether or not some cathodic reduction occurs immediately after starting the measurement. For the process examined in pure H2SO4 (Fig. 4, curve 1), no current was recorded from the rest potential (0.06 V) to the vertex potential 0.05 V and then until the potential 0.21 V was reached. The completed curve is composed of a mirror set of intercalation – deintercalation peaks. The left cathodic peak corresponds to the decomposition of dilute H2SO4 –GIC (high-
stages GIC) to pristine graphite. On the voltammogram obtained in the electrolyte containing the MoO3 additive (Fig. 4, curve 2), the cathodic peaks appear instantly after the measurement is initiated. The peaks well match those recorded during the second cycle in the electrolyte free of MoO3. This result proves without any doubt that before starting the potential sweep, higher stages of GIC are chemically formed due to the oxidation of graphite by molybdenum compounds present in the electrolyte. The lateral anodic peaks on curve 2 are quite similar in charge and position to those noted without MoO3 additive (curve 1), which means that electrochemical formation of the corresponding stages of GIC occurs in the same manner. In the centre of the voltammogram, there is the large and broad peak. It can be pointed out with ease that the oxidation reactions involving molybdenum compounds contribute to this peak. However, due to high amplitude of this peak, it is difficult to distinguish between the reactions occurring in the solution and in the graphite lattice (the peaks corresponding to the latter reactions are hidden under the broad peak arising from the former ones). The reversibility of the deintercalation – intercalation process in the shortened potential range is, however, very satisfactory and the total charge is over twice larger than that for the process taking place in pure H2SO4. From comparison of voltammograms recorded in the whole and shortened range of potentials (Figs. 2 and 4), it is clear that the mechanism of cathodic reactions occurring at poten-
Fig. 4. Cyclic voltammograms recorded for a graphite flake bed electrode in (1) 18 M H2SO4 (- -) and (2) 18 M H2SO4 containing 0.5 M MoO3 (—). Potential range: 0.05 X 0.50 V. Scan rate: 4 AV s1.
J.M. Skowron´ski / Solid State Ionics 157 (2003) 51–55
tials lower than 0.5 V undergoes modification if the precedent anodic polarization up to 1.1 V followed by the reduction down to 0.5 V is carried out. Because no features arising from the presence of molybdenum compounds in the electrolyte were observed on the anodic side of voltammogram between 0.5 and 1.1 V, hence the broadening of muliple cathodic peak AV, precedent by the double cathodic peak BV(0.6 V), cannot only be accounted for by the reduction of molybdenum compounds at the graphite/solution interface but results also from deintercalation processes involving the phase transition of intercalation domains encompassing HSO 4 and MoOd 3 intercalates. At this stage of investigation it is not possible to determine the chemical composition of molybdenum compounds taking part in the intercalation process. It is, however, likely that the interaction of MoOd 3 with HSO intercalate brings about the complex ions 4 which are capable of intercalating. Schulz et al. [9] have reported that the chemical process of intercalation in MoO3/CCl3COOH solution can successively occur provided the active system MoO3/CCl3COCl is formed.
4. Conclusions The intercalation process occurring in H2SO4/ MoO3 solution is interesting as a new reversible sys-
55
tem for chemical power sources. As compared to the process observed in pure acid, the discharge (deintercalation)/charge (intercalation) reversibility is as high, whereas the specific discharge capacity is over 1.5 times higher in the presence of MoO3.
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