A low-cost disposal pencil electrode for cyclic voltammetry analysis

Pooja Chandravanshi¹, Ankita Tejwani¹, Tushar Kant², Manas Kanti Deb¹, and Kamlesh Shrivas¹*

¹School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur-492010, CG, India

²Shaheed Kawasi Rodda Pedda, Govt. College Kuakonda, Dantewada-494552, CG, India

Abstract.

This study explores the use of disposable pencil electrodes in cyclic voltammetry (CV) and compares their performance to traditional glassy carbon electrodes. We conducted CV tests to evaluate different pencil grades, HB, 2B, 4B, 6B, 8B, and 10B along with glassy carbon to assess response time, peak current, and electrode stability during the analysis of potassium ferrocyanide. The results showed that the 4B pencil outperformed other grades and the glassy carbon electrode, with faster response times, higher peak currents, and greater stability. Notably, the 4B pencil's anodic and cathodic currents were higher at 25 mV/s scan rate. This work demonstrates that 4B pencils can serve as cost-effective and reliable alternatives to glassy carbon electrodes in electrochemical applications, offering an accessible option for potassium ferrocyanide analysis and broader electrochemical studies.

Keywords: Pencil electrode, electrochemical response, potassium ferrocyanide, cyclic voltammetry, electrode material.

Introduction

Electrochemical reactions involve the transfer of electrons between chemical species, leading to changes in their oxidation states. These reactions underpin key processes like energy storage, corrosion, and catalysis. They occur within electrochemical cells, where electrodes drive redox reactions, converting chemical energy into electrical energy, or vice versa. Voltammetry is a technique used to study how current response to applied potential in these systems. It encompasses a variety of methods, including cyclic voltammetry (CV), differential pulse voltammetry (DPV), square wave voltammetry (SWV), and linear sweep voltammetry (LSV). These techniques provide valuable data on the kinetics of electron transfer among electroactive species in solution (Elgrishi, Rountree et al. 2018, Rana, Baig et al. 2019).

In CV, the potential applied to an electrode is linearly cyclically swept between two fixed values while measuring the current flow. The resulting plot of current versus potential is known as a cyclic voltammogram. CV is a versatile technique used across various fields like analytical chemistry, materials science, electroplating, and the development of electrochemical sensors and biosensors (Tavares and Barbeira 2008, King, Friend et al. 2010, Marken, Neudeck et al. 2010, Dilgin, Kızılkaya et al. 2012). It provides crucial insights into the behavior of electroactive species in solution. A typical CV setup includes a potentiostat to control the potential applied to the working electrode and a three-electrode cell comprising a working electrode, a reference electrode, and a counter electrode. The working electrode is where the electrochemical reaction occurs; it is typically made of conductive materials like platinum, gold, or glassy carbon electrode (GCE) and can be customized with various materials such as nanoparticles, polymers, or enzymes for specific purposes (Mabbott 1983, Rusling and Suib 1994, Menshykau, Streeter et al. 2008, Chan, Koch et al. 2017, Cook, Ko et al. 2023). A GCE is a type of carbon-based electrode with a highly ordered structure that resembles glass in terms of hardness and resistance. It offers high chemical stability, low porosity, and good conductivity, making it ideal for electrochemical applications. However, GCE are costly, rigid, and have limited surface area compared to other carbon-based electrodes, such as graphite pencil electrodes. The reference electrode, often a saturated calomel electrode (SCE) or a silver/silver chloride electrode (Ag/AgCl), maintains a stable potential against which the working electrode's potential is compared. The counter electrode provides a current pathway, completing the electrical circuit (Evans, O'Connell et al. 1983, Kissinger and Heineman 1983, Grenthe, Stumm et al. 1992, Hagfeldt and Graetzel 1995, 2013, Lakhera, Chaudhary et al. 2022). This setup allows researchers to study redox processes, evaluate material properties, and develop applications for sensors and biosensors.

Graphite, known for its unique combination of metallic and nonmetallic properties, serves as an excellent electrode material. Graphite is a naturally occurring carbon mineral with a hexagonal lattice structure, commonly used in various industries due to its unique properties. Graphite is non-toxic, chemically stable, and resistant to corrosion, oxidation, and most chemical reactions. Graphite is an excellent conductor of electricity and heat, used in electronics for electrodes, brushes, and collectors. Its high thermal conductivity and anisotropic structure, which varies in strength depending on the measurement direction, add to its versatility. Graphite pencil electrode (GPE) are popular for their low cost, ease of use, commercial availability, and disability (Annu, Sharma et al. 2020).

GPE have been utilized in a range of innovative applications. For instance, Purushottam et al. devised a method for analyzing chlorpromazine using GPE. Kevin C et al. used GPE for the detection of Pb²⁺ in anodic stripping voltammetry, demonstrating an eco-friendly, cost-effective alternative for electrochemical devices. Tadi et al. employed molecularly imprinted polymer (MIP) technology for sulfanilamide (SN) sensing, using in-situ electro-polymerization of pyrrole on pencil graphite electrodes. Heydari et al. developed a hydrazine sensor by depositing copper nanostructures (CuNs) on modified GPE. These examples illustrate the versatility and cost-effectiveness of GPE in a wide range of electrochemical applications (Annu, Sharma et al. 2020, da Silva, Paschoalino et al. 2020).

This study aims to assess the suitability of disposable pencil electrodes for CV compared to GCE. Evaluating different pencil grades alongside glassy carbon, it identifies 4B pencils as superior, offering faster response times, higher peak currents, and greater stability, suggesting their potential as cost-effective alternatives in electrochemical analysis.

Materials and Methods

Chemical and reagent

All the chemicals used were of analytical reagent (AR) grade. Potassium ferrocyanide K₄[Fe (CN)₆], (98.0%), potassium chloride (KCl, 99.5%), were purchased from HIMEDIA (Mumbai, India). Double distilled water (DW) was used for the preparation of standard solutions and the cyclic voltammetry (CV) analysis. Phosphate buffer solutions (PBS) of pH 7.0 and 9.0 were prepared from NaH₂PO₄ (99.0%) and Na₂HPO₄ (98.0%) obtained from HIMEDIA (Mumbai, India).

Apparatus

A digital multimeter was used for the measurement of resistance in different types of HB pencils and Potentiostat (Squidstat Solo, Admiral Instrument, USA) was employed for CV measurements of K₄[Fe (CN)₆].

Preparation of different pencil electrodes

To prepare pencil electrodes for electrochemical applications, the pencils with different graphite concentrations, such as HB, 2B, 4B, 6B, 8B, and 10B were selected for present investigations (Nixon and Parry 1968, Kopelevich and Esquinazi 2007). These grades vary in hardness and brightness, with H indicating hardness and B indicating softness. The sharp blade was used to carefully remove of wooden casing to expose the graphite lead. To control the exposed area, tightly wrap the remaining pencil with Teflon tape, leaving only the intended electrode surface open to the solution. After preparation, test the pencil electrode in a standard solution to confirm its stability and functionality, adjusting as needed to ensure a consistent and reliable electrode for your electrochemical applications.

Electrochemical analysis of potassium ferrocyanide using GPE

For the electrochemical measurement of K₄[Fe (CN)₆] using CV with pencil electrodes, follow this methodology. The select pencils with varying graphite grades, including HB, 2B, 4B, 6B, 8B, and 10B, to use as working electrodes. Commercially obtained electrodes, specifically a silver/silver chloride (Ag/AgCl) electrode as the reference and a platinum electrode as the counter, should also be employed. In the electrochemical cell, prepare a phosphate-buffered saline (0.2M PBS, pH=7.0) solution with potassium chloride (KCl, 1.0 M) as the supporting electrolyte. Insert the pencil electrode into the electrolyte solution alongside the reference and counter electrodes. Ensure that only the desired length of the pencil electrode's graphite is exposed to the solution to control the electrode's surface area. The set up the potentiostat for CV, ensuring correct connections to the working, reference, and counter electrodes. The CV measurements were done by sweeping the potential linearly within a predetermined range and at a specific scan rate to observe the redox behavior of K₄[Fe (CN)₆]. Record the resulting CV to analyze the electrochemical responses and determine the electrochemical characteristics of the pencil electrodes.

Result and Discussion

Electrochemical study of K₄[Fe (CN)₆] in CV using GPE

In this study, we explored the electrochemical activity of different pencil electrodes in the CV measurement of K₄[Fe (CN)₆]. Various types of graphite pencils, HB, 2B, 4B, 6B, 8B, and 10B were used as working electrodes, as depicted in Fig. 1 (a and b). Additionally, commercially sourced Ag/AgCl and platinum electrodes were used as the reference and counter electrodes, respectively.  To conduct the experiment, a three-electrode setup was employed, consisting of a graphite pencil electrode as the working electrode, an Ag/AgCl electrode as the reference, and a platinum electrode as the counter. This setup was connected to a potentiostat from Admiral Instruments, allowing precise control and measurement of the applied potential and resultant current, shown in Fig. 2. The electrochemical cell contained a 0.1 M solution of potassium ferrocyanide K₄[Fe (CN)₆], with 1 M KCl as the supporting electrolyte and 0.2 M PBS to maintain a stable pH environment. The working electrode was immersed in the solution along with the reference and counter electrodes, ensuring proper contact and positioning. CV measurements were performed by sweeping the potential across a specified range while recording the resulting current.

Fig. 1. (a and b) illustrate the different types of graphite pencils electrodes (HB, 2B, 4B, 6B, 8B, and 10B)

Fig. 2. Scheme for three electrode systems for CV measurement of (K₄[Fe(CN)₆]).

    Fig. 3 shows the anodic and cathodic current peaks in CV measurements of K₄[Fe (CN)₆]. This technique was used to evaluate the redox behavior of the ferrocyanide compound and to assess the performance of different pencil electrodes.  In CV, the redox reactions occurring at the cathode and oxidation at anode. For the given reaction involving (𝐹𝑒(𝐶𝑁)₆)⁴⁻ and (Fe (CN)₆​)³⁻, the reduction at the cathode and oxidation at the anode, respectively can be described by the following half-reactions:

Reduction at cathode:

When the potential was changed from 1.2 V to −0.75 V, the reduction occurred at the cathode at 0.1 V. In CV, the reduction reaction at the cathode typically occurs at potentials slightly below the standard reduction potential due to the applied potential and reaction kinetics.

(𝐹𝑒(𝐶𝑁)₆)⁴⁻+𝑒⁻→(𝐹𝑒(𝐶𝑁)₆)³⁻

Oxidation at anode:

When the potential was changed from −0.75 V to 1.2 V, the oxidation occurred at the anode at 0.6 V. In CV, the oxidation reaction at the anode typically occurs at potentials slightly above the standard reduction potential due to the applied potential and reaction kinetics.

(𝐹𝑒(𝐶𝑁)₆)³⁻→(𝐹𝑒(𝐶𝑁)₆)⁴⁻+𝑒⁻

In summary, in CV, reduction at the cathode typically occurs at potentials slightly below the standard reduction potential, and oxidation at the anode typically occurs at potentials slightly above the standard reduction potential, due to the applied potential and reaction kinetics.

                                                            Potential (Volt)

Fig. 3. CV peaks of using of (K₄[Fe (CN)₆] using 4B pencil electrode as a working electrode at 25 mV/s

 Electrochemical performance of GPE for measurement of K₄[Fe (CN)₆] in CV

Anodic and cathodic currents in CV refer to the flow of electrons during oxidation and reduction reactions, respectively. Anodic current is generated when the potential applied to the working electrode causes oxidation, driving electrons from the electrode to the solution. Cathodic current is observed when the potential drives electrons from the solution to the electrode, indicating reduction (Tasić, Mihajlović et al. 2022). Based on the given Table 1, the trends in anodic and cathodic currents for different types of pencil electrodes at various scan rates (25, 50, 100 mV/s) can be analyzed.

The anodic current represents the flow of electrons from the electrode to the solution during oxidation. In general, the anodic current increases as the scan rate increases, indicating that higher scan rates lead to more rapid oxidation (Deepa, Madhu et al. 2021). Among the different pencil types, 4B shows the highest anodic current at the 25 mV/s scan rate (0.0051 A), with a similar trend at 50 mV/s (0.007 A). However, at the 100 mV/s scan rate, 4B's anodic current drops significantly (0.0013 A). This fluctuation might suggest a change in conductivity or resistance at higher scan rates. Thus, 25 mV/s scan rate was found for analysis of K₄[Fe (CN)₆] using 4B pencil as a working electrode in anodic current analyses.

Further, cathodic reflects the flow of electrons from the solution to the electrode during reduction. Similar to the anodic current, cathodic currents generally increase with higher scan rates (Shama, Asir et al. 2020). At 25 mV/s, the cathodic current for 4B (-0.0054 A) is much higher than other pencils, indicating a strong reduction process. However, at 100 mV/s, the cathodic current for 4B drops significantly (-0.0101 A), suggesting potential changes in electrode behavior at higher scan rates. Herein, also 25 mV/s scan rate was found for analysis of K₄[Fe (CN)₆] using 4B pencil as a working electrode in cathodic analyses. Fig. 4. Shows the anodic and cathodic currents at at different scanning rate (a)25mV/s, (b) 50mV/s, and (c) 100mV/s using different types of GPE in CV analsyes.

Overall, the table illustrates that scan rates and electrode type play crucial roles in determining the anodic and cathodic currents. It also suggests that 4B is the most effective pencil type at 25 mV/s scan rate.

Fig. 4. Anodic and cathodic currents at different scanning rates (a) 25mV/s, (b) 50 mV/s, and (c) 100 mV/s using different types of GPE in CV analysis

Electrochemical performance of GPE with size, diameter, and resistance in the measurement of K₄[Fe (CN)₆] in CV

Anodic and cathodic currents in electrochemical studies represent the flow of electrons during oxidation and reduction processes, respectively. In CV, the anodic current refers to electron flow from the electrode to the solution (oxidation), while the cathodic current is electron flow from the solution to the electrode (reduction) (Devkota, Chuangchote et al. 2021, Kayali, Shama et al. 2023). The values from your given table reflect these currents at a scanning rate of 25 mV/s for various pencil types, Shown in Table 2.

This current occurs when the electrode facilitates oxidation reactions. The higher the anodic current, the greater the oxidation activity at the electrode. According to table, the 4B pencil exhibits the highest anodic current (0.0051 A), indicating it is the most efficient for oxidation reactions at this scan rate. The other pencils show lower anodic currents, with HB being the lowest at 0.0009 A. This trend suggests that 4B offers better conductive properties or a more favorable structure for electron flow during oxidation.

This current is observed during reduction reactions, when electrons move from the solution to the electrode. As with anodic currents, the 4B pencil leads with the highest cathodic current (-0.00542 A), indicating strong reduction activity. The other pencils display lower cathodic currents, with HB again being the lowest (-0.00026 A), suggesting that 4B's structure and composition are optimal for reduction reactions.

The table also provides resistance, radius, and diameter information, which can influence the anodic and cathodic currents. Notably, the resistance tends to decrease with increasing diameter and radius, with the exception of 2B, which has a higher resistance. This decrease in resistance may be why 4B shows higher current values the lower the resistance, the easier it is for electrons to flow, resulting in stronger anodic and cathodic currents.

In summary, the table indicates that among the pencils tested, the 4B type has the highest anodic and cathodic currents at a 25 mV/s scan rate, suggesting it is the most efficient for both oxidation and reduction reactions. This is likely due to a combination of its optimal radius, diameter, and lower resistance compared to other pencil types.

Comparision of performance of GCE and GPE in CV analyses

GPE can produce higher anodic and cathodic currents for potassium ferrocyanide (K₄[Fe (CN)₆]) in CV compared to GCEs due to their unique structure and composition, shown in Fig. 5. Graphite, with its layered structure, provides a high density of delocalized electrons, enhancing electron transfer efficiency during redox reactions. This property can lead to greater interaction with the electroactive species in CV. Additionally, the larger surface area and flexibility of GPEs, compared to the rigid structure of GCEs, offer more contact points with the electrolyte, facilitating greater electron flow. The combination of these factors can result in higher anodic and cathodic currents with GPE compared to GCEs in CV analysis of potassium ferrocyanide.

Fig. 5. CV peaks of K₄[Fe (CN)₆] in CV when (a) Glassy carbon electrode (b) 4B Pencil electrode

Conclusions

This research focused on the use of GPE in CV, comparing their performance to traditional  GCEs in the analysis of potassium ferrocyanide (K₄[Fe(CN)₆]). The study revealed that GPE, particularly the 4B grade, exhibited superior electrochemical characteristics, with higher anodic and cathodic currents, faster response times, and greater stability compared to GCEs. The novelty lies in demonstrating that GPE are a cost-effective and flexible alternative to GCEs for electrochemical applications. These findings suggest that GPE can be valuable in diverse electrochemical studies, offering an accessible, disposable, and efficient option for researchers and practitioners.

 Acknowledges

Kamlesh Shrivas acknowledged the  Department of Science & Technology (DST)-Promotion of University Research and Scientific Excellence (SR/PURSE/2022/145) and Chhattisgarh Council of Science and Technology (Ref. No: CCOST/EMR/2023), Raipur for providing the financial assistance.

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