Interfacial charge storage mechanisms of composite electrodes based on poly(ortho-phenylenediamine)/carbon nanotubes via advanced electrogravimetry

To reach a deeper understanding of the charge storage mechanisms of electrode materials is one of the challenges toward improving their energy storage performance. Herein, we investigate the interfacial ion exchange of a composite electrode made of carbon nanotube/poly( ortho-phenylenediamine) (CNT/P oPD) in a 1M NaCl aqueous electrolyte via advanced electrogravimetric analyses based on electrochemical quartz crystal microbalance (EQCM). Classical EQCM at different scan rates of the potential revealed the complex electrogravimetric behavior likely due to multi-species participation at different temporal scales. Thereafter, in order to better understand the behavior of each species (ions, counter ions, and co-ions) in the charge compensation mechanism, the electrogravimetric impedance spectroscopy analysis (also called ac-electrogravimetry) was pursued. Ac-electrogravimetry revealed the role of each species where Na+ cations and Cl− anions as well as protons participate in the charge compensation mechanism of the CNT/P oPD composite with different kinetics and proportions. The water molecules with opposite flux direction with the cations are also detected, suggesting their exclusion during cationic species transfer. Having analyzed ac-electrogravimetry responses in depth, the synergistic interaction between the CNT and P oPD is highlighted, revealing the improved accessibility of species to new sites in the composite.

Aqueous Multivalent Charge Storage Mechanism in Aromatic Diamine-Based Organic Electrodes

Rechargeable batteries employing aqueous electrolytes are more reliable and cost-effective as well as possess high ionic conductivity compared to the flammable organic electrolyte solutions. Among these types of batteries, aqueous batteries with multivalent ions attract more attention in terms of providing high energy density. Herein, electrochemical behavior of an organic electrode based on a highly aromatic polymer containing 2,3-diaminophenazine repeating unit, namely poly(ortho-phenylenediamine) (PoPD), is tested in two different multivalent ions (Zn²⁺ and Al³⁺) containing aqueous electrolytes, that is, in zinc sulfate and aluminum chloride solutions. PoPD is synthesized via electropolymerization, and its ion transport and storage mechanism are comprehensively investigated by structural and electrochemical analyses. The electrochemical quartz crystal microbalance, time-dependent Fourier transform infrared, and electrochemical impedance spectroscopy analyses as well as ex situ X-ray diffraction observations established that along with the Zn²⁺ or Al³⁺ ions, reversible proton insertion/extraction also takes place. Contrary to the most of the organic electrodes that requires the use of conductive carbon additives, the electrodeposited PoPD electrode is intrinsically electrically conductive enough, resulting in a binder and additive free electrode assembly. In addition, its discharge products do not dissolve in aqueous medium. As a whole, the resulting PoPD electrode delivers excellent rate performances with prolonged cycle life in which discharge capacities of ∼110 mAh g^-1 in 0.25 M AlCl₃ and ∼93 mAh g^-1 in 1 M ZnSO₄ aqueous electrolyte after 1000 cycles at a current density of 5C have been achieved.