Impedantiometric behavior of solid biopolymer electrolyte elaborated from cassava starch synthesized in different pH

This paper approaches the study of pH’s effect on elaborate films of solid biopolymer electrolyte from cassava starch and its impedantiometric response. The films of solid biopolymer electrolyte were elaborated by thermochemical synthesis while varying the pH (2, 4, 5, 7, 9, 10, and 12). Starch was extracted from cassava tubers by a traditional method (disintegrated, washed, decanted, filtered, and dried). Solid biopolymer electrolyte films were processed by thermochemical synthesis  by adding plasticizers (glycerol, glutaraldehyde, and polyethylene glycol) and lithium salt (lithium perchlorate). The impedance behavior was studied using the electrochemical impedance spectroscopy technique. The Nyquist and Bode’s diagrams registered presented a similar trend in all the films; therefore, they were described by the same equivalent circuit model. However, the equivalent circuit components presented different values in each case. The conductivity and capacitance showed a quadratic polynomial tendency in relation to the pH, obtaining the highest conductivity in the films elaborated at acidic pH and the highest capacitance in the films elaborated at basic pH.  The degree of basicity or acids allowed conductivity to be modulated or capacitance of the solid biopolymer electrolyte as required. It could be concluded that the production pH has a marked effect on impedantiometric behavior of films of solid biopolymer electrolyte from cassava starch, which may be useful to modulate the electrochemical properties of this type of material in future Applications. 

Cassava; Electrochemical impedance; pH; Solid biopolymer electrolyte; Starch

Within the wide range of materials available, polymers are undoubtedly the most used in the development of an ample variety of utensils and devices for everyday use. Polymers are very popular due to their excellent mechanical properties, high resistance to attack by organic and inorganic solvents, and corrosion resistance, among others. The global use of polymers is so great that they have now become an environmental problem worldwide, as they constitute one of the main sources of pollution (Zhong et al., 2019; Dwivedi et al., 2019; Emi-Cassola et al., 2019). 

    Polymers are considered highly polluting materials (Dwivedi et al., 2019; Emi-Cassola et al., 2019; Shonnard et al., 2019). This contingency has carried numerous research centers worldwide to seek alternatives to replace synthetic polymers (petrochemical origin) worldwide to seek alternatives to replace synthetic polymers (petrochemical origin) with polymers elaborated from biological sources (biopolymers). Biopolymers can have similar properties to synthetic polymers, so they can be used in a wide range of applications and are also friendly to the environment, due to their good biodegradability, low production cost and originating from renewable sources (Mohamed et al., 2018).
    Due to their high technological and industrial potential, the most used and studied biopolymers are cellulose, alginate, starch, chitosan, among others (Rochardjo et al., 2021; Imani et al., 2022; Jyothi et al., 2019). However, the low electrical conductivity of conventional polymers and biopolymers has limited their applications in various new electronic devices. In recent decades, the development of polymers capable of conducting electric current (i.e., conducting polymers) has opened the possibility of applying such materials in applications as diverse as smart windows, solar cells, sensors, artificial muscles, capacitors, electrochemical accumulators, electroluminescent diodes (LEDs), touch panels, among others (Cichosz et al., 2018; Itik et al., 2015; AL-Barani et al., 2019).
    Conducting polymers are divided into two groups; the intrinsic conducting polymers (ICP), which can conduct electric current across the chains with conjugated bonds and charges generated by oxidation or reduction (Awuzie, 2017; Chen et al., 2021). On the other hand, the ionic conducting polymers, which have fixed charges in their chains and mobile ions that produce the electroneutrality in the matrix, in this type of materials the movement of mobile ions gives the conduction, so they are known as polymeric solid electrolytes (Aziz et al., 2018; Angell, 2019). Intrinsic and ionic conducting polymers are mostly synthetic and therefore cause  environmental problems typical of polymers from a petrochemical origin.
    Few works report the synthesis of biopolymers capable of conducting electric current (Arrieta et al., 2011; Mobarak et al., 2015). Recently, it has been reported the use of cassava starch to generate a conducting biopolymer with the use of plasticizers and lithium salt (Arrieta et al., 2011). This biopolymer has been studied as a solid electrolyte for application in an artificial muscle and as an electrochemical accumulator (Núñez et al., 2016; Arrieta et al., 2019a). However, not many studies have been conducted about the effect of synthesized pH on the electrochemical properties of this biopolymeric solid electrolyte. The effect of the synthesized pH on voltametric response and mechanical properties of this type of biopolymer material was reported recently, showing that factors such as redox potentials (oxidation/reduction), crystallinity, voltametric stability, modulus of elasticity and electrical conductivity can be affected. (Arrieta et al., 2019b; Arrieta et al., 2018). In this work, the study of a synthesized pH effect (pH values; 2, 4, 5, 7, 9, 10, and 12) on a conducting biopolymer elaborated from cassava starch about its impedance behavior is presented.

Films of solid biopolymer electrolytes can be elaborated using cassava starch. The films were stable against handling; however, the films synthesized at pH 2 values were brittle and broke during handling. Therefore, the films presented stability when they were elaborated at a pH higher than 4. The impedaciometric behavior of the films showed a similar trend in all cases, is defined by a similar equivalent circuit model. However, the values of the  equivalent circuit components were different in each case. The pH used during the synthesis process affects the electrochemical properties of cassava starch solid biopolymer electrolyte films. The conductivity in the films presented a polynomial relationship (degree 2). The films elaborated at low pH registered a higher conductivity. On the other hand, the capacitance of the films showed a behavior opposite to the conductivity, being higher in the films synthesized at a more basic pH. This behavior could be due to the variation in the crystallinity of the biopolymeric films, which is influenced by the synthesized pH. The conduction mechanisms in solid biopolymer electrolyte films are not affected by pH; however, their capacitive and electrical conduction capacity are altered. In this way, it was determined that the variation in the synthesized pH allows to modulate the properties of conductivity or capacitance according to the application to which the solid biopolymer electrolyte is intended. In future works, the application of this material in smart fertilizer release systems will be studied.

        The authors acknowledgment to The Ministry of Science, Technology, and Innovation (Minciencias) - Colombia for the financial support provided to the project code BPIN 2020000100027 through resources from the General System of Royalties (SGR).

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