Unique Mechanisms of Ion Storage in Polyaniline Electrodes for Pseudocapacitive Energy Storage Devices Unraveled by EQCM-D Analysis

Abstract

The optimal performance of organic electrodes for aqueous batteries requires their full compatibility with selected electrolyte solutions. Electrode materials having 1–3-dimensional structures of variable rigidity possess a confined space in their structure filled with water and electrolyte solutions. Depending on the rigidity and confined space geometry, insertion and extraction of ions into electrode structures are often coupled with incorporation/withdrawal of water molecules. Aside from the scientific interest in understanding the charging mechanism of such systems, co-insertion of solvent molecules affects strongly the charge storage capability of the electrodes for energy storage devices. We present herein in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) investigations of polyaniline (PANI) electrodes operating in various aqueous Na+-containing electrolytes, namely, Na2SO4, NaClO4, NaBF4, and NaPF6. Careful analysis of the EQCM-D results provides a dynamic snapshot of the mixed anionic/protonic fluxes and the accompanying water molecules’ insertion/extraction to/from the PANI electrodes. Based on our observations, it was found that the charging mechanism, as well as the capacity values, strictly depends on the electrolyte pH, the chaotropic/kosmotropic character of the anionic dopants, and the amount of the extracted water molecules. This study demonstrates the effectiveness of analysis by EQCM-D in selecting electrolytes for batteries comprising organic electrodes.

To what extent do anions affect the electrodeposition of Zn?

“…For the former situation (pure gravimetric behavior) the changes in the measured responses are solely corresponding to the variation of mass on the quartz crystal (QC) surface. In this case, the dissipation values remain constant during the application of potential, and the variation in the frequency values (Δ f ) are independent by the overtone number – n In this scenario, the amount of the irreversible mass deposited on the QC can be found by comparing the theoretical frequency (Δ f theo ) calculated from the combined Faraday and Sauerbrey equations and the experimentally measured frequency changes Δ f exp (see more details in our previous papers 21,22 ). Specifically for Zn redox processes Δ f exp > Δ f theo implies irreversible formation of by-products on the QC substrate.…”

“…Df n n ¼ const : In this scenario, the amount of the irreversible mass deposited on the QC can be found by comparing the theoretical frequency (Df theo ) calculated from the combined Faraday and Sauerbrey equations and the experimentally measured frequency changes Df exp (see more details in our previous papers 21,22 ). Specically for Zn redox processes Df exp > Df theo implies irreversible formation of by-products on the QC substrate.…”

Conductive Polymer/Graphene‐based Composites for Next Generation Energy Storage and Sensing Applications

“…The first oxidation process from leucoemeraldine to emeraldine form of PANI results in a formation of electron‐deficient quinonoid structure, accompanied by the extraction of protons and insertion of the electrolyte anions into the PANI chain. Further shift towards more positive potentials results in a coupling of the polymer chain deprotonation with anion extraction, which counteracts the formation of a highly oxidized form of PANI known as pernigraniline [26] . However, the structure of PANI deteriorates owing to the repeating counter‐ion insertion/deinsertion process, causing substantial volume changes and thereby poor cyclic stability, which is unfavorable for energy storage applications [27,28] …”

“…Further shift towards more positive potentials results in a coupling of the polymer chain deprotonation with anion extraction, which counteracts the formation of a highly oxidized form of PANI known as pernigraniline. [26] However, the structure of PANI deteriorates owing to the repeating counter-ion insertion/deinsertion process, causing substantial volume changes and thereby poor cyclic stability, which is unfavorable for energy storage applications. [27,28] To overcome these limitations of PANI, binary composites have been proposed that combine CPs and graphene materials (including GO and rGO) to achieve high-performance electrode materials with remarkable electrochemical performance and long-term stability.…”

Proton is Essential or Not: A Fresh Look on Pseudocapacitive Energy Storage of PANI Composites

“…[ 21 ] A low pH generally indicates a high level of protonation, allowing for a high conductivity of PANI. [ 22 ] In strong acids (pH = 1–3), for instance, the initial protonated leucoemeraldine salt (LS, conducting) transforms into a highly conductive protonated emeraldine salt (ES 2+ , conducting), and then a protonated pernigraniline salt (PNS) form upon further oxidation. In weakly acidic solutions (pH = 4–5), the nonprotonated form of leucoemeraldine base (LB, insulating) transforms into protonated ES (less conductive than ES 2+ ), forming pernigraniline base (PNB, insulating).…”