- Email: [email protected]
A potential working electrode based on graphite and montmorillonite for electrochemical applications in both aqueous and molten salt electrolytes
Electrochemistry Communications 108 (2019) 106562
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
A potential working electrode based on graphite and montmorillonite for electrochemical applications in both aqueous and molten salt electrolytes
T
Kohobhange S.P. Karunadasaa, C.H. Manoratnea, H.M.T.G.A. Pitawalab, R.M.G. Rajapaksec,
⁎
a
Materials Technology Section, Industrial Technology Institute, No. 363, Bauddhaloka Mawatha, Colombo 7, Sri Lanka Department of Geology, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka c Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka b
ARTICLE INFO
ABSTRACT
Keywords: Graphite Montmorillonite Composite electrode Enhanced thermal stability Electrochemical applications
The feasibility of a novel graphite–MMT composite electrode for electrochemical processes in aqueous and molten salt electrolytes has been investigated. The graphite–MMT composite electrodes (G-MMTCEs) were fabricated by preparing composites in deionized water, pressing the dry composite under 1.03 × 104 N ram force to obtain cylindrical electrodes (5.00 cm long and 1.00 cm in diameter) and firing the electrodes at around 550 °C for 1 h. The results indicate that the G-MMTCE containing 80% graphite showed the lowest resistivity (8.17 × 10−4 Ωm) and highest flexural strength (5.81 × 106 Nm2). The exponential decrease in resistivity from low to high graphite percentage is clearly observed for a series of G-MMTCEs. The lamella-like graphite structure held together by tiny clay particles accounts for the enhanced electrical and mechanical stability of the GMMTCEs. It is also found that the fabricated G-MMTCE is very stable in molten salts as well as in aqueous electrolytes with different pH values. The G-MMTCE has advantages as a working electrode over a glassy carbon electrode (GCE) in analyte detection as well as in electropolymerization. A narrow working potential range and enhanced sensitivity are the major advantages of the G-MMTCE over the GCE under identical cell and measurement conditions.
1. Introduction The replacement of expensive metallic electrodes with cheaper composite materials can be considered an interesting topic that opens the gateway to the cutting-edge technology of graphite-based electrodes. Graphite-based electrode technology has developed significantly since the fabrication of the first carbon paste electrode by Ralph N. Adams in 1958 [1]. Although the method of electrode fabrication is quite simple, involving only the mixing of graphite with suitable binders, the performance of the electrode depends largely on the type of binder, as well as on various modifications (incorporation of polymers, redox mediators, and recognition elements [2,3]) and doping agents (clay [4], carbon nanotubes [5] and metal nanoparticles [6–8], etc.). The advantages of graphite-based electrodes are well known: easy fabrication, selectivity enhancement through various modifications, low cost, renewable surfaces and hazard-free cleaning [9]. However, there are also certain weaknesses yet to be addressed: confinement to narrow temperature ranges (poor thermal stability, especially at high temperatures), mechanical failures in molten salt electrolytes, rapid aging, limited reproducibility and durability, etc. [10]. Performance
⁎
enhancement of graphite-based electrodes is usually achieved either by changing the binder type or by modifying existing binders [10]. However, organic binders, including ionic liquids, eventually limit the working temperature range of the electrode [10]. Binder-free graphite electrode technology has recently emerged (e.g. graphite-cement electrode); however, the technology is still being developed for specific applications [11]. Therefore, the aim of this study is to fabricate a stable composite electrode (free from organic binders) based on graphite and montmorillonite that has the potential to be used over a wide range of temperatures in both aqueous and molten salt electrolytes. 2. Material and methods Natural graphite (> 99.9%, average particle size 39.1 μm) and purified montmorillonite clay, MMT (> 95%, average particle size 4.2 μm) were obtained from the Department of Geology, University of Peradeniya, Sri Lanka. The background electrolytes, KCl (99.5%), H2SO4 (95%) and HCl (≥37%), were obtained from Sigma-Aldrich Ltd. The primary analytes, ammonium ferrous sulfate hexahydrate [(NH4)2Fe(SO4)2·6H2O, 99%] and aniline (99.5%), were acquired from
Corresponding author at: Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya 20400, Sri Lanka. E-mail address: [email protected] (R.M.G. Rajapakse).
https://doi.org/10.1016/j.elecom.2019.106562 Received 27 August 2019; Received in revised form 3 October 2019; Accepted 3 October 2019 Available online 15 October 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
Sigma-Aldrich Ltd. and AnalaR, respectively. Anhydrous CaCl2 (99%, MP Biomedicals LLC) was used as the molten salt electrolyte. First, the thermal profile and stability of the raw materials were determined by TGA/DSC analysis (TA instruments SDTQ600). The average particle size was also determined using a FRITSCH Analysette 22 NanoTec particle size analyzer. The graphite–MMT composites were prepared by mixing different ratios of graphite and MMT (20:80, 40:60, 50:50, 60:40 and 80:20) in deionized water, followed by continuous stirring for 1 h at 1100 rpm using a magnetic stirrer (VELP Scientifica). Cylindrical electrodes (5 cm long and 1 cm in diameter) were produced by pressing 8.50 g of dry composite material (under a ram force of 1.03 × 104 N) in a specially designed stainless steel mould using a manual hydraulic press (CARVER 3853-0). The mechanically compressed electrodes were then fired at around 550 °C for 1 h in a tube furnace (ELITE TMH 12/75/750). The resistance of the fabricated electrodes (G-MMTCE) was determined by linear sweep voltammetry (Biologic SP 150 Potentiostat/Galvanostat) in which the current values were plotted against the scanning potential (potential range 0 to 0.45 V, scanning rate 20 mV s−1). The resistivity (ρ) was estimated using the equation below [12]:
=
RA l
(1)
where ρ, R, A, and l are the resistivity, resistance, cross-sectional area (7.86 × 10−5 m2) and length of the electrode (5 cm), respectively. The modulus of rupture (σ) or flexural strength of the fabricated electrodes was determined using a Com-ten DFM 5000 tester (three-point bending system). A cylindrical electrode of each composition was then employed in MOR analysis. The flexural strength/stress (σ) of a circular cross-sectional electrode can be calculated using the following equation [13]:
=
FL r3
(2)
Fig. 1. TGA-DSC thermograms of (a) graphite; (b) MMT.
where F, L, and r are the force at the fracture point, length of the supporting span, and radius of the electrode (0.5 cm), respectively. The cross-sectional morphology of the electrodes was also studied with a scanning electron microscope (LEO 1420VP). The G-MMTCE with 80% graphite was employed in a cyclic voltammetry analysis to determine its possible use as a working electrode. The cyclic voltammetry analysis was carried out using a Biologic SP 150 Potentiostat/Galvanostat, with a platinum wire (ALS.Co.Ltd) and a saturated calomel electrode (ALS.Co.Ltd) as the counter and reference electrodes, respectively. The electrochemical performance of the GMMTCE was compared with that of a typical glassy carbon electrode (ALS.Co.Ltd, cross-sectional area 7.1 × 10−6 m2). A cyclic voltammetry analysis of freshly prepared ammonium ferrous sulfate hexahydrate (50 mmol dm−3) in acidified (using 1.0 × 10−2 mol dm−3 HCl) 1 mol dm−3 KCl as the background electrolyte was carried out to evaluate the analyte detection competency of the G-MMTCE. The potential of the G-MMTCE for the electropolymerization of organic monomers was studied using 20 mmol dm−3 aniline solution (in 1 mol dm−3 sulfuric acid as the background electrolyte). The measurement conditions employed in the cyclic voltammetry experiments
Fig. 2. G-MMTCE with (a) 20%; (b) 40%; (c) 50%; (d) 60% and (e) 80% of graphite.
are displayed in Table 1. The cyclic voltammograms were obtained under nitrogen-saturated conditions, with a 30-minute saturation period prior to analysis. The characterization and parameter evaluation related to the cyclic voltammetry were carried out using EC Lab software interfaced with the Biologic SP 150 potentiostat/galvanostat.
Table 1 Measurement conditions for cyclic voltammetry analysis at room temperature. Method
Analyte
WE
Identification of electroactive species (analyte detection)
Ammonium ferrous sulfate hexahydrate
Electro-polymerization
Aniline
WE – Working electrode, GCE – glassy carbon electrode. 2
GCE G-MMTCE GCE G-MMTCE
Measurement conditions Peak potential (V)
Scan speed (mV s−1)
Number of scans
−1.0 −0.2 −1.0 −1.0
25.0 25.0 20.0 20.0
3 3 9 9
to to to to
1.5 1.0 1.0 1.4
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
Fig. 3. Electrical and mechanical properties of the G-MMTCE series (a) resistance (linear sweep voltammetry analysis); (b) resistivity (ρ) and flexural strength (σ).
Furthermore, the stability of the fabricated electrodes in aqueous solutions with different pH values was studied by immersing a small piece of the electrode in an acid solution (pH = 1, HCl), distilled water (pH = 7) and an alkaline solution (pH = 13, NaOH). The stability was tested after prolonged exposure (for 1 month) of the electrode to the above solutions. The electrical and mechanical stability of G-MMTCEs in molten salt electrolytes (for high-temperature applications) was determined by dipping the electrodes in a molten bath of CaCl2 (in alumina crucibles) at 800 °C for 3 h. The resistivity of the G-MMTCEs before and after the molten salt treatment was compared using linear sweep voltammetry to examine the effect of the molten salt treatment on the electrical properties of the G-MMTCEs.
3.1. Characterization of the G-MMTCEs
Fig. 4. SEM micrographs of the horizontal cross-section of (a) Electrode with 20% graphite (magnification ×2000), the clay particles (tiny white spots bound to graphite sheets) are randomly distributed throughout the electrode matrix where the agglomerates can be seen due to higher adherence between clay particles; (b) Electrode with 80% graphite (magnification ×2000), uniform and even distribution of very tiny clay particles (white spots, most likely in the nano range) can be observed throughout the matrix; (c) Lamella-like arrangement of graphite sheets under the influence of very tiny MMT particles (scattered white spots) where the numbers 1 (top sheet) and 2 (bottom sheet) depict the adjacent carbon sheets in the lamella-like structure (electrode with 80% graphite, magnification ×5000). This structure improves the electrical conductivity and strength of the electrode.
3.1.1. Electrical and mechanical properties (resistivity and flexural strength) A series of G-MMTCEs with different compositions was fabricated in order to check the correlation between the amount of clay/graphite and the electrical/mechanical stability of the electrode. The firing temperature of the electrode was determined by taking both the aerial oxidation of graphite and dehydroxylation of MMT into consideration. It was therefore set below the temperature of the aerial oxidation of graphite (600 °C) and well above the first dehydroxylation temperature
(500 °C) of MMT. The fired G-MMTCEs showed improved mechanical stability and water resistance (Fig. 1). The series of G-MMTCEs is shown in Fig. 2. It is important to select the most suitable composition for a G-MMTCE, bearing in mind both its electrical (resistivity) and mechanical (flexural strength) stability in order to prevent vital failures during operation. A significant decrease in resistivity can be clearly observed as the amount of graphite in the composite increases, with the G-MMTCE containing 80% graphite
3. Results and discussion
3
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
Table 2 Resistivity of a series of G-MMTCEs before and after molten salt treatment. Resistivity × 10−3 (Ωm)
Wt% of graphite
20 40 50 60 80
+ 1.84 × 10
101 3.96 2.97 2.33 0.82
101 3.95 2.95 2.30 0.80
Table 3 Cyclic voltammogram data related to analyte detection (with either G-MMTCE or GCE as the working electrode).
showing the lowest resistivity (Fig. 3). It appears that the resistivity of the G-MMTCE series decreases exponentially according to the relationship given below: w /5.33)
After
Fig. 6. Cyclic voltammograms of ammonium ferrous sulfate hexahydrate (cycle number 2) with (a) GCE as working electrode; (b) G-MMTCE as working electrode, SCE – saturated calomel electrode.
Fig. 5. Stability of G-MMTCE (electrode with 80% graphite) at different pH values: (a) 0.1 M HCl (pH = 1); (b) distilled water (pH = 7) and (c) 0.1 M NaOH (pH = 13).
= 4.23e (
Before
Peak characteristic
3
where ρ and w (0 < w < 1 0 0) are the resistivity and percentage weight of graphite, respectively (see Fig. 3b). This correlation also indicates that the incorporation of a small amount of MMT has very little effect on resistivity compared to the electrodes with a higher percentage of MMT. Unlike the resistivity, the flexural strength of G-MMTCE
Peak potential (V) Current (mA) Positive charge (mC) Negative charge (mC)
4
Peak I (Oxidation)
Peak II (Reduction)
GCE
G-MMTCE
GCE
G-MMTCE
1.261 0.164 1.385 0
0.596 23.82 173.7 0
−0.243 0.094 0 −0.904
0.292 23.71 0 −150.7
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
Furthermore, the dual action of a clay mineral as a self-binder and matrix entrapment agent avoids the need for expensive organic/inorganic binders in the fabrication of G-MMTCEs. The existing literature has reported the fabrication of graphite–MMT electrodes using conventional paraffin binders; however, due to the low stability of the binder, they are limited to low temperature applications [14]. Therefore, the present work can be seen as a first attempt to fabricate binderfree graphite–MMT electrodes for both low and high temperature applications.
Table 4 Sensitivity of the working electrode in terms of current density. Working electrode
GCE G-MMTCE
Current density (A m−2) Oxidation
Reduction
23.19 23.35
13.29 23.24
Average current density (A m−2) 18.24 23.29
series increases as the percentage of graphite increases. Although the greatest flexural strength might reasonably be expected for electrodes containing the largest amount of MMT, the G-MMTCE series rather showed the opposite behavior, with the highest flexural strength observed for an electrode with a significantly low amount of MMT (Fig. 3b). Unlike the exponential decrease in resistivity, the flexural strength increases roughly linearly with the increasing amount of graphite (Fig. 3b). The considerable increase in flexural strength can be explained by considering the internal arrangement of the electrode matrix. The SEM images clearly reveal the formation of very tiny clay particles (most are in the nano range) between graphite layers, which are responsible for strongly held lamella-like graphite structure (Fig. 4). The significant increase in flexural strength can be explained by considering the number of clay particles, the uniformity in clay particle distribution within the electrode matrix and the relative size of clay particles, which presumably determine the level of electrical contact between adjoining graphite sheets. During mechanical compression the externally applied ram force initiates the formation of spherical clay particles under the lubricating effect of graphite. The irregular large raw clay particles tend to roll in between the graphite sheets under mechanical compression; while rolling under the lubricating effect of graphite the particles decrease significantly in size and become more spherical in shape. However, the size of the clay particles is most likely determined by the amount of clay included in the composite. It is quite obvious that a large amount of clay material between the graphite sheets can reduce the rolling movement of the clay particles to some extent, due to steric hindrance, thus avoiding the substantial reduction in particle size seen in electrodes with a low percentage of clay content (agglomeration is more likely than size reduction due to strong particle adherence). Conversely, a low clay percentage permits more freedom in the rolling movement of the clay particles as there is less steric hindrance and weaker particle adherence (weak particle agglomeration). Small spherical clay particles are associated with the strongest lamellalike structure in the electrode containing the lowest MMT percentage, which also has the highest flexural strength and lowest resistivity. The improved electrical contact between adjoining graphite sheets is presumably due to the strongly held lamella-like structure containing small clay particles. This structure explains why the electrode with 80% graphite (Fig. 3b) has significantly low resistivity and high flexural strength. It is hoped that this type of technique, where mechanical compression is used to produce tiny clay particles (preferably in the nano range) under lubrication, will be recognised as an advance in clay nanoparticles synthesis and adopted in future research.
3.1.2. Stability in both aqueous and molten salt electrolytes The fabricated G-MMTCE is very stable in both aqueous and molten salt electrolytes (CaCl2). Fig. 5 illustrates the prolonged exposure of the G-MMTCE to different pH solutions (at room temperature). After a month, the electrodes remained intact, without undergoing surface degradation or fracture formation. It is also hard to observe any significant change in resistivity or noticeable damage to the electrode matrix following molten salt treatment at 800 °C for a period of 3 h (Table 2). 3.2. G-MMTCE as a potential working electrode (cyclic voltammetry analysis) 3.2.1. Analyte detection An electrode containing 80% graphite was used in a cyclic voltammetry analysis to determine its potential as a working electrode. A cyclic voltammogram recorded in 50 mmol dm−3 ammonium ferrous sulfate hexahydrate clearly indicates the oxidation (peak I) and reduction (peak II) of the Fe2+/Fe3+ system at redox potentials which are low compared to those observed with a glassy carbon electrode (Fig. 6). Compared with the redox potentials obtained with a GCE (peak I and peak II in Fig. 6a), the G-MMTCE is associated with much lower oxidation/reduction potentials that result in a narrower working potential range (0.292–0.596 V) compared to the GCE (−0.243 V to 1.261 V) (Fig. 6b and Table 3). The narrow working potential range implies that the G-MMTCE is more effective in aqueous electrolytes than the GCE. The detection of analytes in aqueous electrolytes can be more easily achieved with a G-MMTCE since it is possible to narrow down the working potential range of a given analyte that lies within the water window (the potentials responsible for electrolysis of water). This indicates the advantage of the G-MMTCE over the GCE for analyte detection even at low potential ranges (within water window) in aqueous electrolytes. However, very high peak currents (both oxidation and reduction) are observed due to the large geometric surface area of the G-MMTCE (effective surface area immersed in the electrolyte solution, 1.02 × 10−3 m2) compared with the GCE (7.07 × 10−6 m2, electrode diameter 3.0 mm) (Table 3). The sensitivity of the two electrodes towards the Fe2+/Fe3+ system can be predicted based on current densities since the analyte concentration is similar in magnitude (50 mmol dm−3). The average current densities obtained for both oxidation and reduction are given in Table 4 (the current density is calculated by dividing the total oxidation/reduction current by the effective surface area). It can be seen that the average sensitivity of the G-
Table 5 Cyclic voltammogram data related to electropolymerization of aniline. Peak characteristic
Oxidation
Reduction
Peak I
Peak potential (V) Current (mA) Positive charge (mC) Negative charge (mC)
Peak II
Peak III
Peak IV
GCE
G-MMTCE
GCE
G-MMTCE
GCE
G-MMTCE
GCE
G-MMTCE
0.435 0.887 4.827 0
0.311 19.91 41.05 0
0.713 0.031 0.037 0
0.703 31.03 241.3 0
0.220 0.178 0 −0.661
0.289 34.56 0 −233.6
−0.204 0.411 0 −1.363
−0.126 5.30 0 −25.74
5
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
MMTCE toward the Fe2+/Fe3+ system is somewhat higher than that of GCE (Table 4). 3.2.2. Electropolymerization The possible use of the G-MMTCE as a working electrode for electropolymerization of organic monomers is studied using 20 mmol dm−3 aniline solution. The voltammograms reveal that the fabricated GMMTCE can be successfully employed for electropolymerization of organic monomers at a lower potential range compared to the GCE (Table 5 and Fig. 7). The oxidation (peaks 1 and 2) and reduction (peaks 3 and 4) potentials observed during the electropolymerization of aniline using the G-MMTCE are somewhat lower than the potentials obtained with the GCE as the working electrode (Table 5). This is similar to the observations made regarding analyte detection (Section 3.2.1). The uniform polyaniline coating is also clearly visible on the GMMTCE surface together with increased oxidation and reduction currents during a series of potential scans, demonstrating the successful electropolymerization of aniline (Fig. 7c). The broader peaks are typically attributed to high current densities resulting from the large active geometric surface area of the G-MMTCE compared to the GCE (Fig. 7). The ability to prepare highly stable polymer coatings on the G-MMTCE surface demonstrates the possibility of fabricating modified electrodes and sensors that have a wide range of demand-driven electrochemical applications. 4. Conclusions The present work describes a novel graphite–MMT composite electrode that can potentially be employed for different electrochemical applications in both aqueous and molten salt electrolytes. These binderfree electrodes are fabricated using a simple process where mechanical compression and dehydroxylation (firing) play a significant role in property enhancement. It is found that the electrode with a higher graphite percentage (80%) shows the lowest resistivity and highest flexural strength. The mechanical stability of the G-MMTCE is enhanced by incorporating clay minerals (MMT) which also play the role of a binder. The formation of very tiny MMT clay particles (preferably in the nano range) in between graphite layers under applied ram force accounts for the strongly held lamella-like structure that further increases the mechanical strength across the electrode both horizontally and vertically. This special structure also contributes to higher electrical conductivity despite the fact that MMT itself is an insulator. The clay matrix also prevents the rapid aging of the electrode even under prolonged usage (better reusability) due to its improved chemical and mechanical resistance. It is shown that the fabricated G-MMTCEs are stable in both aqueous and molten salt electrolytes. This stability at high temperatures suggests that the G-MMTCE is a prospective candidate as an anode material in mineral deoxidation and other metallurgical processes. The stability in different pH solutions also explains the improved chemical resistance of the G-MMTCE. The present study further revealed that the G-MMTCE containing 80% graphite has better electrochemical performance than the GCE for analyte detection and electropolymerization. The low potential ranges and improved sensitivity of G-MMTCE for analyte detection are key attributes that support its preferential use instead of GCE as a successful working electrode. The improved electropolymerization capability of G-MMTCE also shows that it is feasible to fabricate high-tech sensors and modified electrodes on demand. Fig. 7. Electropolymerization of aniline using different working electrodes (a) GCE; (b) G-MMTCE; (c) Polyaniline-coated G-MMTCE (uniform polyaniline coating on G-MMTCE surface).
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 6
K.S.P. Karunadasa, et al.
Electrochemistry Communications 108 (2019) 106562
Acknowledgement
[6] S. Liu, J. Yu, H. Ju, J. Electroanal. Chem. 540 (2003) 61–67. [7] L. Agüí, J. Manso, P. Yáñez-Sedeño, J.M. Pingarrón, Sens. Actuators, B 113 (2006) 272–280. [8] H. Heli, M. Hajjizadeh, A. Jabbari, A.A. Moosavi-Movahedi, Biosens. Bioelectron. 24 (2009) 2328–2333. [9] S.A. Wring, J.P. Hart, Analyst 117 (1992) 1215–1229. [10] D. Bellido-Milla, L.M. Cubillana-Aguilera, M. El Kaoutit, M.P. Hernández-Artiga, J.L. Hidalgo-Hidalgo de Cisneros, I. Naranjo-Rodríguez, J.M. Palacios-Santander, Anal. Bioanal. Chem. 405 (2013) 3525–3539. [11] Y. Yuan, H. Li, S. Han, L. Hu, G. Xu, Talanta 84 (2011) 49–52. [12] C.C. Okpoli, Int. J. Geophys. 2013 (2013) 608037. [13] J.S. Temenoff, A.G. Mikos, Biomaterials: The Intersection of Biology and Materials Science, First US edn., Pearson Prentice Hall, New Jersey, 2008. [14] Y. Kong, Y. Xu, H. Mao, C. Yao, X. Ding, J. Electroanal. Chem. 669 (2012) 1–5.
This work was supported by the Industrial Technology Institute (Treasury Grants, TG 16/127 & TG 19/181) of Sri Lanka. References [1] [2] [3] [4] [5]
R.N. Adams, Anal. Chem. 30 (1958) 1576–1576. J. Wang, J. Liu, G. Cepra, Anal. Chem. 69 (1997) 3124–3127. L. Gorton, Electroanalysis 7 (1995) 23–45. C. Mousty, Appl. Clay Sci. 27 (2004) 159–177. S. Shahrokhian, L. Fotouhi, Sens. Actuators, B 123 (2007) 942–949.
7
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
A potential working electrode based on graphite and montmorillonite for electrochemical applications in both aqueous and molten salt electrolytes
T
Kohobhange S.P. Karunadasaa, C.H. Manoratnea, H.M.T.G.A. Pitawalab, R.M.G. Rajapaksec,
⁎
a
Materials Technology Section, Industrial Technology Institute, No. 363, Bauddhaloka Mawatha, Colombo 7, Sri Lanka Department of Geology, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka c Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka b
ARTICLE INFO
ABSTRACT
Keywords: Graphite Montmorillonite Composite electrode Enhanced thermal stability Electrochemical applications
The feasibility of a novel graphite–MMT composite electrode for electrochemical processes in aqueous and molten salt electrolytes has been investigated. The graphite–MMT composite electrodes (G-MMTCEs) were fabricated by preparing composites in deionized water, pressing the dry composite under 1.03 × 104 N ram force to obtain cylindrical electrodes (5.00 cm long and 1.00 cm in diameter) and firing the electrodes at around 550 °C for 1 h. The results indicate that the G-MMTCE containing 80% graphite showed the lowest resistivity (8.17 × 10−4 Ωm) and highest flexural strength (5.81 × 106 Nm2). The exponential decrease in resistivity from low to high graphite percentage is clearly observed for a series of G-MMTCEs. The lamella-like graphite structure held together by tiny clay particles accounts for the enhanced electrical and mechanical stability of the GMMTCEs. It is also found that the fabricated G-MMTCE is very stable in molten salts as well as in aqueous electrolytes with different pH values. The G-MMTCE has advantages as a working electrode over a glassy carbon electrode (GCE) in analyte detection as well as in electropolymerization. A narrow working potential range and enhanced sensitivity are the major advantages of the G-MMTCE over the GCE under identical cell and measurement conditions.
1. Introduction The replacement of expensive metallic electrodes with cheaper composite materials can be considered an interesting topic that opens the gateway to the cutting-edge technology of graphite-based electrodes. Graphite-based electrode technology has developed significantly since the fabrication of the first carbon paste electrode by Ralph N. Adams in 1958 [1]. Although the method of electrode fabrication is quite simple, involving only the mixing of graphite with suitable binders, the performance of the electrode depends largely on the type of binder, as well as on various modifications (incorporation of polymers, redox mediators, and recognition elements [2,3]) and doping agents (clay [4], carbon nanotubes [5] and metal nanoparticles [6–8], etc.). The advantages of graphite-based electrodes are well known: easy fabrication, selectivity enhancement through various modifications, low cost, renewable surfaces and hazard-free cleaning [9]. However, there are also certain weaknesses yet to be addressed: confinement to narrow temperature ranges (poor thermal stability, especially at high temperatures), mechanical failures in molten salt electrolytes, rapid aging, limited reproducibility and durability, etc. [10]. Performance
⁎
enhancement of graphite-based electrodes is usually achieved either by changing the binder type or by modifying existing binders [10]. However, organic binders, including ionic liquids, eventually limit the working temperature range of the electrode [10]. Binder-free graphite electrode technology has recently emerged (e.g. graphite-cement electrode); however, the technology is still being developed for specific applications [11]. Therefore, the aim of this study is to fabricate a stable composite electrode (free from organic binders) based on graphite and montmorillonite that has the potential to be used over a wide range of temperatures in both aqueous and molten salt electrolytes. 2. Material and methods Natural graphite (> 99.9%, average particle size 39.1 μm) and purified montmorillonite clay, MMT (> 95%, average particle size 4.2 μm) were obtained from the Department of Geology, University of Peradeniya, Sri Lanka. The background electrolytes, KCl (99.5%), H2SO4 (95%) and HCl (≥37%), were obtained from Sigma-Aldrich Ltd. The primary analytes, ammonium ferrous sulfate hexahydrate [(NH4)2Fe(SO4)2·6H2O, 99%] and aniline (99.5%), were acquired from
Corresponding author at: Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya 20400, Sri Lanka. E-mail address: [email protected] (R.M.G. Rajapakse).
https://doi.org/10.1016/j.elecom.2019.106562 Received 27 August 2019; Received in revised form 3 October 2019; Accepted 3 October 2019 Available online 15 October 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
Sigma-Aldrich Ltd. and AnalaR, respectively. Anhydrous CaCl2 (99%, MP Biomedicals LLC) was used as the molten salt electrolyte. First, the thermal profile and stability of the raw materials were determined by TGA/DSC analysis (TA instruments SDTQ600). The average particle size was also determined using a FRITSCH Analysette 22 NanoTec particle size analyzer. The graphite–MMT composites were prepared by mixing different ratios of graphite and MMT (20:80, 40:60, 50:50, 60:40 and 80:20) in deionized water, followed by continuous stirring for 1 h at 1100 rpm using a magnetic stirrer (VELP Scientifica). Cylindrical electrodes (5 cm long and 1 cm in diameter) were produced by pressing 8.50 g of dry composite material (under a ram force of 1.03 × 104 N) in a specially designed stainless steel mould using a manual hydraulic press (CARVER 3853-0). The mechanically compressed electrodes were then fired at around 550 °C for 1 h in a tube furnace (ELITE TMH 12/75/750). The resistance of the fabricated electrodes (G-MMTCE) was determined by linear sweep voltammetry (Biologic SP 150 Potentiostat/Galvanostat) in which the current values were plotted against the scanning potential (potential range 0 to 0.45 V, scanning rate 20 mV s−1). The resistivity (ρ) was estimated using the equation below [12]:
=
RA l
(1)
where ρ, R, A, and l are the resistivity, resistance, cross-sectional area (7.86 × 10−5 m2) and length of the electrode (5 cm), respectively. The modulus of rupture (σ) or flexural strength of the fabricated electrodes was determined using a Com-ten DFM 5000 tester (three-point bending system). A cylindrical electrode of each composition was then employed in MOR analysis. The flexural strength/stress (σ) of a circular cross-sectional electrode can be calculated using the following equation [13]:
=
FL r3
(2)
Fig. 1. TGA-DSC thermograms of (a) graphite; (b) MMT.
where F, L, and r are the force at the fracture point, length of the supporting span, and radius of the electrode (0.5 cm), respectively. The cross-sectional morphology of the electrodes was also studied with a scanning electron microscope (LEO 1420VP). The G-MMTCE with 80% graphite was employed in a cyclic voltammetry analysis to determine its possible use as a working electrode. The cyclic voltammetry analysis was carried out using a Biologic SP 150 Potentiostat/Galvanostat, with a platinum wire (ALS.Co.Ltd) and a saturated calomel electrode (ALS.Co.Ltd) as the counter and reference electrodes, respectively. The electrochemical performance of the GMMTCE was compared with that of a typical glassy carbon electrode (ALS.Co.Ltd, cross-sectional area 7.1 × 10−6 m2). A cyclic voltammetry analysis of freshly prepared ammonium ferrous sulfate hexahydrate (50 mmol dm−3) in acidified (using 1.0 × 10−2 mol dm−3 HCl) 1 mol dm−3 KCl as the background electrolyte was carried out to evaluate the analyte detection competency of the G-MMTCE. The potential of the G-MMTCE for the electropolymerization of organic monomers was studied using 20 mmol dm−3 aniline solution (in 1 mol dm−3 sulfuric acid as the background electrolyte). The measurement conditions employed in the cyclic voltammetry experiments
Fig. 2. G-MMTCE with (a) 20%; (b) 40%; (c) 50%; (d) 60% and (e) 80% of graphite.
are displayed in Table 1. The cyclic voltammograms were obtained under nitrogen-saturated conditions, with a 30-minute saturation period prior to analysis. The characterization and parameter evaluation related to the cyclic voltammetry were carried out using EC Lab software interfaced with the Biologic SP 150 potentiostat/galvanostat.
Table 1 Measurement conditions for cyclic voltammetry analysis at room temperature. Method
Analyte
WE
Identification of electroactive species (analyte detection)
Ammonium ferrous sulfate hexahydrate
Electro-polymerization
Aniline
WE – Working electrode, GCE – glassy carbon electrode. 2
GCE G-MMTCE GCE G-MMTCE
Measurement conditions Peak potential (V)
Scan speed (mV s−1)
Number of scans
−1.0 −0.2 −1.0 −1.0
25.0 25.0 20.0 20.0
3 3 9 9
to to to to
1.5 1.0 1.0 1.4
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
Fig. 3. Electrical and mechanical properties of the G-MMTCE series (a) resistance (linear sweep voltammetry analysis); (b) resistivity (ρ) and flexural strength (σ).
Furthermore, the stability of the fabricated electrodes in aqueous solutions with different pH values was studied by immersing a small piece of the electrode in an acid solution (pH = 1, HCl), distilled water (pH = 7) and an alkaline solution (pH = 13, NaOH). The stability was tested after prolonged exposure (for 1 month) of the electrode to the above solutions. The electrical and mechanical stability of G-MMTCEs in molten salt electrolytes (for high-temperature applications) was determined by dipping the electrodes in a molten bath of CaCl2 (in alumina crucibles) at 800 °C for 3 h. The resistivity of the G-MMTCEs before and after the molten salt treatment was compared using linear sweep voltammetry to examine the effect of the molten salt treatment on the electrical properties of the G-MMTCEs.
3.1. Characterization of the G-MMTCEs
Fig. 4. SEM micrographs of the horizontal cross-section of (a) Electrode with 20% graphite (magnification ×2000), the clay particles (tiny white spots bound to graphite sheets) are randomly distributed throughout the electrode matrix where the agglomerates can be seen due to higher adherence between clay particles; (b) Electrode with 80% graphite (magnification ×2000), uniform and even distribution of very tiny clay particles (white spots, most likely in the nano range) can be observed throughout the matrix; (c) Lamella-like arrangement of graphite sheets under the influence of very tiny MMT particles (scattered white spots) where the numbers 1 (top sheet) and 2 (bottom sheet) depict the adjacent carbon sheets in the lamella-like structure (electrode with 80% graphite, magnification ×5000). This structure improves the electrical conductivity and strength of the electrode.
3.1.1. Electrical and mechanical properties (resistivity and flexural strength) A series of G-MMTCEs with different compositions was fabricated in order to check the correlation between the amount of clay/graphite and the electrical/mechanical stability of the electrode. The firing temperature of the electrode was determined by taking both the aerial oxidation of graphite and dehydroxylation of MMT into consideration. It was therefore set below the temperature of the aerial oxidation of graphite (600 °C) and well above the first dehydroxylation temperature
(500 °C) of MMT. The fired G-MMTCEs showed improved mechanical stability and water resistance (Fig. 1). The series of G-MMTCEs is shown in Fig. 2. It is important to select the most suitable composition for a G-MMTCE, bearing in mind both its electrical (resistivity) and mechanical (flexural strength) stability in order to prevent vital failures during operation. A significant decrease in resistivity can be clearly observed as the amount of graphite in the composite increases, with the G-MMTCE containing 80% graphite
3. Results and discussion
3
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
Table 2 Resistivity of a series of G-MMTCEs before and after molten salt treatment. Resistivity × 10−3 (Ωm)
Wt% of graphite
20 40 50 60 80
+ 1.84 × 10
101 3.96 2.97 2.33 0.82
101 3.95 2.95 2.30 0.80
Table 3 Cyclic voltammogram data related to analyte detection (with either G-MMTCE or GCE as the working electrode).
showing the lowest resistivity (Fig. 3). It appears that the resistivity of the G-MMTCE series decreases exponentially according to the relationship given below: w /5.33)
After
Fig. 6. Cyclic voltammograms of ammonium ferrous sulfate hexahydrate (cycle number 2) with (a) GCE as working electrode; (b) G-MMTCE as working electrode, SCE – saturated calomel electrode.
Fig. 5. Stability of G-MMTCE (electrode with 80% graphite) at different pH values: (a) 0.1 M HCl (pH = 1); (b) distilled water (pH = 7) and (c) 0.1 M NaOH (pH = 13).
= 4.23e (
Before
Peak characteristic
3
where ρ and w (0 < w < 1 0 0) are the resistivity and percentage weight of graphite, respectively (see Fig. 3b). This correlation also indicates that the incorporation of a small amount of MMT has very little effect on resistivity compared to the electrodes with a higher percentage of MMT. Unlike the resistivity, the flexural strength of G-MMTCE
Peak potential (V) Current (mA) Positive charge (mC) Negative charge (mC)
4
Peak I (Oxidation)
Peak II (Reduction)
GCE
G-MMTCE
GCE
G-MMTCE
1.261 0.164 1.385 0
0.596 23.82 173.7 0
−0.243 0.094 0 −0.904
0.292 23.71 0 −150.7
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
Furthermore, the dual action of a clay mineral as a self-binder and matrix entrapment agent avoids the need for expensive organic/inorganic binders in the fabrication of G-MMTCEs. The existing literature has reported the fabrication of graphite–MMT electrodes using conventional paraffin binders; however, due to the low stability of the binder, they are limited to low temperature applications [14]. Therefore, the present work can be seen as a first attempt to fabricate binderfree graphite–MMT electrodes for both low and high temperature applications.
Table 4 Sensitivity of the working electrode in terms of current density. Working electrode
GCE G-MMTCE
Current density (A m−2) Oxidation
Reduction
23.19 23.35
13.29 23.24
Average current density (A m−2) 18.24 23.29
series increases as the percentage of graphite increases. Although the greatest flexural strength might reasonably be expected for electrodes containing the largest amount of MMT, the G-MMTCE series rather showed the opposite behavior, with the highest flexural strength observed for an electrode with a significantly low amount of MMT (Fig. 3b). Unlike the exponential decrease in resistivity, the flexural strength increases roughly linearly with the increasing amount of graphite (Fig. 3b). The considerable increase in flexural strength can be explained by considering the internal arrangement of the electrode matrix. The SEM images clearly reveal the formation of very tiny clay particles (most are in the nano range) between graphite layers, which are responsible for strongly held lamella-like graphite structure (Fig. 4). The significant increase in flexural strength can be explained by considering the number of clay particles, the uniformity in clay particle distribution within the electrode matrix and the relative size of clay particles, which presumably determine the level of electrical contact between adjoining graphite sheets. During mechanical compression the externally applied ram force initiates the formation of spherical clay particles under the lubricating effect of graphite. The irregular large raw clay particles tend to roll in between the graphite sheets under mechanical compression; while rolling under the lubricating effect of graphite the particles decrease significantly in size and become more spherical in shape. However, the size of the clay particles is most likely determined by the amount of clay included in the composite. It is quite obvious that a large amount of clay material between the graphite sheets can reduce the rolling movement of the clay particles to some extent, due to steric hindrance, thus avoiding the substantial reduction in particle size seen in electrodes with a low percentage of clay content (agglomeration is more likely than size reduction due to strong particle adherence). Conversely, a low clay percentage permits more freedom in the rolling movement of the clay particles as there is less steric hindrance and weaker particle adherence (weak particle agglomeration). Small spherical clay particles are associated with the strongest lamellalike structure in the electrode containing the lowest MMT percentage, which also has the highest flexural strength and lowest resistivity. The improved electrical contact between adjoining graphite sheets is presumably due to the strongly held lamella-like structure containing small clay particles. This structure explains why the electrode with 80% graphite (Fig. 3b) has significantly low resistivity and high flexural strength. It is hoped that this type of technique, where mechanical compression is used to produce tiny clay particles (preferably in the nano range) under lubrication, will be recognised as an advance in clay nanoparticles synthesis and adopted in future research.
3.1.2. Stability in both aqueous and molten salt electrolytes The fabricated G-MMTCE is very stable in both aqueous and molten salt electrolytes (CaCl2). Fig. 5 illustrates the prolonged exposure of the G-MMTCE to different pH solutions (at room temperature). After a month, the electrodes remained intact, without undergoing surface degradation or fracture formation. It is also hard to observe any significant change in resistivity or noticeable damage to the electrode matrix following molten salt treatment at 800 °C for a period of 3 h (Table 2). 3.2. G-MMTCE as a potential working electrode (cyclic voltammetry analysis) 3.2.1. Analyte detection An electrode containing 80% graphite was used in a cyclic voltammetry analysis to determine its potential as a working electrode. A cyclic voltammogram recorded in 50 mmol dm−3 ammonium ferrous sulfate hexahydrate clearly indicates the oxidation (peak I) and reduction (peak II) of the Fe2+/Fe3+ system at redox potentials which are low compared to those observed with a glassy carbon electrode (Fig. 6). Compared with the redox potentials obtained with a GCE (peak I and peak II in Fig. 6a), the G-MMTCE is associated with much lower oxidation/reduction potentials that result in a narrower working potential range (0.292–0.596 V) compared to the GCE (−0.243 V to 1.261 V) (Fig. 6b and Table 3). The narrow working potential range implies that the G-MMTCE is more effective in aqueous electrolytes than the GCE. The detection of analytes in aqueous electrolytes can be more easily achieved with a G-MMTCE since it is possible to narrow down the working potential range of a given analyte that lies within the water window (the potentials responsible for electrolysis of water). This indicates the advantage of the G-MMTCE over the GCE for analyte detection even at low potential ranges (within water window) in aqueous electrolytes. However, very high peak currents (both oxidation and reduction) are observed due to the large geometric surface area of the G-MMTCE (effective surface area immersed in the electrolyte solution, 1.02 × 10−3 m2) compared with the GCE (7.07 × 10−6 m2, electrode diameter 3.0 mm) (Table 3). The sensitivity of the two electrodes towards the Fe2+/Fe3+ system can be predicted based on current densities since the analyte concentration is similar in magnitude (50 mmol dm−3). The average current densities obtained for both oxidation and reduction are given in Table 4 (the current density is calculated by dividing the total oxidation/reduction current by the effective surface area). It can be seen that the average sensitivity of the G-
Table 5 Cyclic voltammogram data related to electropolymerization of aniline. Peak characteristic
Oxidation
Reduction
Peak I
Peak potential (V) Current (mA) Positive charge (mC) Negative charge (mC)
Peak II
Peak III
Peak IV
GCE
G-MMTCE
GCE
G-MMTCE
GCE
G-MMTCE
GCE
G-MMTCE
0.435 0.887 4.827 0
0.311 19.91 41.05 0
0.713 0.031 0.037 0
0.703 31.03 241.3 0
0.220 0.178 0 −0.661
0.289 34.56 0 −233.6
−0.204 0.411 0 −1.363
−0.126 5.30 0 −25.74
5
Electrochemistry Communications 108 (2019) 106562
K.S.P. Karunadasa, et al.
MMTCE toward the Fe2+/Fe3+ system is somewhat higher than that of GCE (Table 4). 3.2.2. Electropolymerization The possible use of the G-MMTCE as a working electrode for electropolymerization of organic monomers is studied using 20 mmol dm−3 aniline solution. The voltammograms reveal that the fabricated GMMTCE can be successfully employed for electropolymerization of organic monomers at a lower potential range compared to the GCE (Table 5 and Fig. 7). The oxidation (peaks 1 and 2) and reduction (peaks 3 and 4) potentials observed during the electropolymerization of aniline using the G-MMTCE are somewhat lower than the potentials obtained with the GCE as the working electrode (Table 5). This is similar to the observations made regarding analyte detection (Section 3.2.1). The uniform polyaniline coating is also clearly visible on the GMMTCE surface together with increased oxidation and reduction currents during a series of potential scans, demonstrating the successful electropolymerization of aniline (Fig. 7c). The broader peaks are typically attributed to high current densities resulting from the large active geometric surface area of the G-MMTCE compared to the GCE (Fig. 7). The ability to prepare highly stable polymer coatings on the G-MMTCE surface demonstrates the possibility of fabricating modified electrodes and sensors that have a wide range of demand-driven electrochemical applications. 4. Conclusions The present work describes a novel graphite–MMT composite electrode that can potentially be employed for different electrochemical applications in both aqueous and molten salt electrolytes. These binderfree electrodes are fabricated using a simple process where mechanical compression and dehydroxylation (firing) play a significant role in property enhancement. It is found that the electrode with a higher graphite percentage (80%) shows the lowest resistivity and highest flexural strength. The mechanical stability of the G-MMTCE is enhanced by incorporating clay minerals (MMT) which also play the role of a binder. The formation of very tiny MMT clay particles (preferably in the nano range) in between graphite layers under applied ram force accounts for the strongly held lamella-like structure that further increases the mechanical strength across the electrode both horizontally and vertically. This special structure also contributes to higher electrical conductivity despite the fact that MMT itself is an insulator. The clay matrix also prevents the rapid aging of the electrode even under prolonged usage (better reusability) due to its improved chemical and mechanical resistance. It is shown that the fabricated G-MMTCEs are stable in both aqueous and molten salt electrolytes. This stability at high temperatures suggests that the G-MMTCE is a prospective candidate as an anode material in mineral deoxidation and other metallurgical processes. The stability in different pH solutions also explains the improved chemical resistance of the G-MMTCE. The present study further revealed that the G-MMTCE containing 80% graphite has better electrochemical performance than the GCE for analyte detection and electropolymerization. The low potential ranges and improved sensitivity of G-MMTCE for analyte detection are key attributes that support its preferential use instead of GCE as a successful working electrode. The improved electropolymerization capability of G-MMTCE also shows that it is feasible to fabricate high-tech sensors and modified electrodes on demand. Fig. 7. Electropolymerization of aniline using different working electrodes (a) GCE; (b) G-MMTCE; (c) Polyaniline-coated G-MMTCE (uniform polyaniline coating on G-MMTCE surface).
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 6
K.S.P. Karunadasa, et al.
Electrochemistry Communications 108 (2019) 106562
Acknowledgement
[6] S. Liu, J. Yu, H. Ju, J. Electroanal. Chem. 540 (2003) 61–67. [7] L. Agüí, J. Manso, P. Yáñez-Sedeño, J.M. Pingarrón, Sens. Actuators, B 113 (2006) 272–280. [8] H. Heli, M. Hajjizadeh, A. Jabbari, A.A. Moosavi-Movahedi, Biosens. Bioelectron. 24 (2009) 2328–2333. [9] S.A. Wring, J.P. Hart, Analyst 117 (1992) 1215–1229. [10] D. Bellido-Milla, L.M. Cubillana-Aguilera, M. El Kaoutit, M.P. Hernández-Artiga, J.L. Hidalgo-Hidalgo de Cisneros, I. Naranjo-Rodríguez, J.M. Palacios-Santander, Anal. Bioanal. Chem. 405 (2013) 3525–3539. [11] Y. Yuan, H. Li, S. Han, L. Hu, G. Xu, Talanta 84 (2011) 49–52. [12] C.C. Okpoli, Int. J. Geophys. 2013 (2013) 608037. [13] J.S. Temenoff, A.G. Mikos, Biomaterials: The Intersection of Biology and Materials Science, First US edn., Pearson Prentice Hall, New Jersey, 2008. [14] Y. Kong, Y. Xu, H. Mao, C. Yao, X. Ding, J. Electroanal. Chem. 669 (2012) 1–5.
This work was supported by the Industrial Technology Institute (Treasury Grants, TG 16/127 & TG 19/181) of Sri Lanka. References [1] [2] [3] [4] [5]
R.N. Adams, Anal. Chem. 30 (1958) 1576–1576. J. Wang, J. Liu, G. Cepra, Anal. Chem. 69 (1997) 3124–3127. L. Gorton, Electroanalysis 7 (1995) 23–45. C. Mousty, Appl. Clay Sci. 27 (2004) 159–177. S. Shahrokhian, L. Fotouhi, Sens. Actuators, B 123 (2007) 942–949.
7