Influence of counterion charge on the electrochemistry and impedance of polypyrrole
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ORIGINAL PAPER
Influence of counterion charge on the electrochemistry and impedance of polypyrrole Abdul-Rahman F. Al-Betar 1 & Peter G. Pickup 2 Received: 17 February 2020 / Revised: 23 March 2020 / Accepted: 24 March 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Polypyrroles doped with multiply charged anions are becoming increasingly important in a wide range of energy, environmental and biomedical applications. The increased counterion charge promotes anion binding and retention and can significantly increase stability and performance. The electrochemical properties of polypyrrole films prepared galvanostatically in Na2SO4 (PPySO4) where found to be similar those of PPyClO4 prepared in NaClO4, although there was significantly more anion retention during potential cycling. In contrast, PPyPO4 films prepared in Na3PO4 under the same conditions were over-oxidised and more dense, which is beneficial for corrosion protection and electroanalysis. Paradoxically, the low mobility of SO42− counterions results in more facile charging and discharging of the film, as observed with large and polymeric counterions. This can create significant benefits in applications that require fast cycling, such as supercapacitors and high-rate batteries. These conclusions are corroborated by electrochemical impedance measurements in various aqueous electrolytes. Ionic conductivity was dominated by anion transport for both PPyClO4 and PPySO4, whilst PPyPO4 was predominantly a cation conductor. Keywords Polypyrrole . Counterion . Ion-exchange . Ion transport . Impedance
Introduction Ion transport in conducting polymers has attracted considerable interest over the past four decades due to its importance in many applications, particularly in the energy [1, 2], environmental [2] and biomedical [3] sectors. Studies of the electrochemical and ion transport properties of polypyrrole have laid the foundation for understanding the influences of synthesis methods, solvents and counterions [4–11], and this has led to materials that can be tailored for each application. The electrochemical switching of conducting polymers between their electronically conducting, p-doped states and insulating un-doped states [10] is generally accompanied by anion (Am−) transfer as shown in Eq. 1, ð−pyn −Þmþ Am− þ me− ⇌ð−pyn −Þ þ Am−
ð1Þ
* Peter G. Pickup [email protected] 1
Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
2
Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X7, Canada
where (−pyn−) represents a segment of a polypyrrole chain and n is typically ca. 4 for m = 1. However, restricted anion mobility can result in cation (C+) insertion during the initial un-doping cycle as shown in Eq. 2, ð−pyn −Þmþ Am− þ me− þ mCþ ⇌ð−pyn −ÞCm A
ð2Þ
The resulting CmA salt can remain strongly associated with the polymer backbone or diffuse out of the polymer film into the supporting electrolyte. These ion and salt transport processes are accompanied
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