Preparation of High Charge Storage Capacity PEDOT/Functionalized Mwcnts Hybrid Nanocomposite for Neural Electrode Applications
Authors: Gunapu D V Santhosh Kumar , Vanjari Siva Rama Krishna 
Published: February 2020

There is an ever growing demand for the development of high charge storage capacity, biocompatible microelectrodes, which are essential for neural recording and stimulation applications. The challenge here is to increase the charge storage capacity. Higher charge density would enable to pump in higher charge at the electrode/biological element interface even in with a smaller area electrode. However, there is a tradeoff between electrode dimension and interface impedance. A decrease in the electrode dimension results in low capacitance and high impedance which is a major drawback for stimulation because higher signal amplitudes are required for efficiently stimulating the cells. In order to overcome this bottleneck, it is imperative to develop materials having higher active area for the same geometric area. This not only reduces the interface impedance but also increases the specific capacitance. In developing these materials, biocompatibility and long term stability considerations must be taken into consideration.

Biocompatible conducting polymers are good candidates for these applications and are being utilized as electrode materials in neuro prosthesis research for the past two decades. Yet the quest of high charge density electrode is still unmet. Among them, PEDOT (Poly Ethylene Dioxy Thiophene) is considered to be one of the promising conducting polymers due to its excellent electronic and ionic conductivity which cater low impedance and high specific capacitance, ease of fabrication by electro chemical polymerization along with excellent biocompatibility and bio stability. But PEDOT suffers from low cycling stability due to its low mechanical strength leading to decrease in pseudo capacitance and conductivity. Choosing the right dopant in PEDOT could potentially overcome these issues.

In this work, we have synthesized different hybrid nanocomposites of PEDOT by doping different types carbon based 2D nanomaterials. The syntheses of these nanocomposites were carried out using electro polymerization. The electrode deposition was carried out using Chrono Amperometry, a widely used Potentiostatic technique. The idea behind using these 2D nanomaterials is well explored owing to the inherent advantages of these materials such as extraordinary mechanical strength, electrical conductivity and high chemical stability. Furthermore, the high surface to volume ratio results in higher surface charge, thereby resulting in a higher storage capacity. The focus of this endeavor is to study the charge storage capacity of the synthesized hybrid nanocomposites.

The dopants that were used to synthesize the nano composites were Graphene nano platelets, Graphene Oxide, Multiwalled Carbon Nanotubes (MWCNTs) and functionalized MWCNTs. Similar conditions were adapted for electro polymerization. All the experiments were carried out at a scan rate of 100 mV/s using CH Instruments electro chemical setup shown in Fig.1. These include finite pulse potential amplitude of 1V and 120s pulse duration. Amongst all the materials, functionalized MWCNTs demonstrated the highest charge storage capacity as can be inferred by the cyclic voltammograms shown in Fig. 2(a). Functionalized CNTs not only disperse well in the precursor solution but also get embedded in a greater quantity in PEDOT matrix owing to their inherent negative charge.

After identifying the right choice of dopant, the next step is to optimize the electro polymerization conditions so as to enhance the charge storage capacity to the best possible value. Cyclic voltammograms (CV) of PEDOT/Functionalized MWCNTs (PEDOT/FCNTs) as shown in Fig. 2(b) reveals that the area of the CV curve is highest for a loading percentage (wt/vol) of 1 mg/ml compared to other loading percentages. We observed that beyond 1 mg/ml, the excess concentration of MWCNTs is impeding the electro polymerization and is not resulting in a good film. The loading was fixed to 1 mg/ml for subsequent experiments. The next parameter that was optimized is the potential of electro polymerization with the duration of electro polymerization being 800s. As can be observed from Fig. 2(c), the suitable potential is 1 V. Voltages beyond this show a reduced charge storage and may be attributed to over oxidation during polymerization. Similarly, Fig. 2(d) reveals that area of the CV curve has attained maximum at pulse duration of 800s by keeping the amplitude as constant at 1V. Fig. 3 depicts the magnitude plot of electrode interface impedance for the PEDOT/CNT-COOH nanocomposite deposited at 1V with pulse duration of 800s in the frequency range of 0.1Hz to 1MHz. Hence, the best possible charge storage capacity and lowest interface impedance is obtained for a ratio of 1mg/ml (wt/vol) with electro polymerization amplitude of 1V and pulse duration 800s. Further morphological studies would be carried out in due of course of time to gain a deeper understanding in the synthesis of hybrid nanocomposite of PEDOT/FCNTs.

Figure 1