posted on 2020-02-12, 20:46authored byLucas
Q. Flagg, Connor G. Bischak, Ramsess J. Quezada, Jonathan W. Onorato, Christine. K. Luscombe, David S. Ginger
We
investigate the mechanism of ion-dependent charge compensation
during electrochemical oxidation (doping) of the model mixed ionic/electronic
transporting polythiophene derivative poly(3-{[2-(2-methoxyethoxy)ethoxy]methyl}thiophene-2,5-diyl)
(P3MEEMT). Using a combination of electrochemical quartz microbalance
gravimetry and glow discharge optical emission spectroscopy, we show
that charge compensation during polymer redox processes proceeds via
a cation-dependent mechanism. For p-type polymer oxidation in certain
electrolytes, charge compensation is achieved by both eventual injection
of anions into the film, as well as initial expulsion of cations from
the film. We compare doping mechanisms for a variety of electrolyte
salts including potassium chloride, tetrabutylammonium chloride, potassium
hexafluorophosphate (KPF6), and tetrabutylammonium hexafluorophosphate.
For the electrolyte KPF6, both the cations and anions coexist
in the water-swelled polymer even prior to application of electrical
bias. Our data indicate that electrochemical doping (hole injection
into the polymer and ionic charge compensation) proceeds via the following
mechanism: (1) hydration of the neutral film by electrolyte (water,
cations, anions), (2) cation (K+) expulsion from the film
upon initial application of an oxidative bias, and (3) anion injection
into the film at higher oxidation/doping levels (>∼2 ×
1020/cm3). Understanding the mechanism of charge
compensation during the doping process should allow for the design
of improved mixed ionic/electronic conductors for use in applications
ranging from organic supercapacitors and redox flow batteries to bioelectronic
sensors, thermoelectrics, and devices for neuromorphic computing.