(Digital Presentation) Capacitive Properties of Electrodes Based on Fto Covered By Silver Nanowires
Authors: Kotok Valerii , Zima Oleksandra , Kovalenko Vadym , Mikolasek Miroslav , Ondrejka Peter 
Published: July 2022

Networks formed with silver nanowires (AgNW) are considered an alternative to conventional transparent conductive oxides – FTO, ITO, AZO, and IZO [1]. Moreover, the AgNW films as flexible support with high conductivity are successfully used. At the same time, the AgNW films can be applied as new electrochemically-active material. It has to be noted that silver-based materials are widely used in the positive electrode of silver-zinc secondary batteries [2]. Following the aforementioned, the AgNW films have a great potential in forming electrodes for transparent flexible micro-batteries. The current work is dedicated to the issues of the AgNW film capacity formed by the spin-coating method.
The formation of the films was carried out on the glass substrates covered by fluorine-doped tin oxide (FTO glass). The surface resistivity of the substrate was ≤10 Ohm/sq (China, Zhuhai Kaivo Optoelectronic Technology Co. Ltd). Before coating, the substrates were treated in several steps. First, the substrates were rubbed with Na2SO4 paste. This was followed by rinsing with running and distilled water, as well as treatment in 96% ethanol under sonication (60 W, 41.5 kHz). The formation of the AgNW films on FTO glass was performed by dropping of few drops on the substrate at different speeds from 0 to 5000 rpm. The rotation speed was measured by laser tachometer UT373 (UNI-T, China). After the forming of the AgNW films, they were dried at room temperature and stored in a dark and dry place before use. For the formation of the AgNW films, the solution in isopropanol with a concentration of 5mg/mL was used (China, commercial source). The parameters of AgNW were an average length of 30µm, and an average diameter of 100 nm. For comparison of electrochemical properties, a silver wire with a diameter of 1.25 mm was used.
For electrochemical measurements we used potentiostat MTech SPG-500L (Ukraine, [0; +1000 mV] vs NHE with sweep rate 1 or 5 mV/s), potentiostat for EIS Palmsens 4 (Netherlands, Step 5 mV, frequency 100 000 – 0.1 Hz) for cyclic voltammetry. Ag|AgCl electrode was employed as a reference electrode. 0.1 M KOH solution was used for Ag and AgNW electrodes cycling.
To estimate the morphology of the AgNW films, an optical microscope (OSEELANG, China) with a camera (Belona, China) and a scanning electron microscope (REM-106I, Ukraine) were used.
A comparison of cyclic voltammetry curves of the AgNW film deposited at 1000 rpm and pure silver showed sharp differences – Figure. The position, shape, and changing of peaks differed for nanosized and macro-sized silver.
It has to be noted that the forming of relatively transparent films requires small AgNW solution volumes. In this case, the mass of the AgNW films on FTO glass especially at high rotating speeds was insufficient for measuring. To find a way of measuring the AgNW film's weight, a series of experiments were carried out where the relation between detectable mass and electrochemical capacity of the electrode was found. During these experiments, the films with a mass from 0.2 to 0.7 mg were formed using the application of several AgNW solution drops without rotating. The plot of the AgNW mass vs capacity derived from cyclic voltammetry curves was linear and can be expressed by the formula C=k*m, where, C is the capacitance of the AgNW film, k is the experimental coefficient and m is the mass of the AgNW film. Further, this dependence was used for the definition of mass for the AgNW films formed at high rotating speeds.
The efficiency of oxidizing and reducing the AgNW films during cathodic and anodic processes was estimated at 26 % of the theoretical one. In addition, according to the scanning electron microscopy images, most of the nanosized wires were split into multiple fragments.

Conclusions

Dependence between the AgNW films' capacities and their mass has been found, which can be used for AgNW mass estimation not detectable by balance.

The efficiency of the AgNW films’ cycling has been estimated, which is 26% of the theoretical one.

It has been shown that during cycling, silver nanowires split into multiple fragments.

Acknowledgment

The authors express their gratitude to the National Scholarship Program of the Slovak Republic and assistance in the program realization to the Slovak Academic Information Agency.

References

Sharma, N., Nair, N. M., Nagasarvari, G. et al (2022). A review of silver nanowire-based composites for flexible electronic applications. Flexible and Printed Electronics, 7(1).

Jeong, J., Lee, J. -W., & Shin, H.-C. (2021). Unique electrochemical behavior of a silver–zinc secondary battery at high rates and low temperatures. Electrochimica Acta, 396.

Figure 1