Article pubs.acs.org/JPCC
Polaron Disproportionation Charge Transport in a Conducting Redox Polymer Hao Huang,† Christoffer Karlsson,† Fikret Mamedov,‡ Maria Strømme,† Adolf Gogoll,§ and Martin Sjödin*,†,∥ †
Nanotechnology and Functional Materials, Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden ‡ Department of Chemistry − Ångström, The Ångström Laboratory, Uppsala University, Box 523, Uppsala, Sweden § Department of Chemistry - BMC, Biomedical Centre, Uppsala University, Box 576, SE-751 23, Uppsala, Sweden ∥ Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan ABSTRACT: Herein we report a mechanistic study of the charge transport in poly-3-((2,5hydroquinone)vinyl)-1H-pyrrole by conductance measurements at various temperatures performed in situ during doping of the polypyrrole backbone in contact with an aqueous electrolyte. Charge transport was found to occur by electron hopping with associated electron transfer activation energies in the range of 0.08−0.2 eV. In situ electron paramagnetic resonance experiments indicated polarons as the dominant charge carriers and the charge transport was found to follow a second-order dependence with respect to the number of accumulated charges. Based on the findings, we present a polaron comproportionation/disproportionation model for electron conduction in poly-3-((2,5-hydroquinone)vinyl)-1H-pyrrole, thus, providing a complement to existing models for charge propagation in conducting polymers.
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INTRODUCTION Since the first discovery of conductive polyacetylene films in the late 1970s,1,2 conducting polymers (CPs) have received great attention in several research fields as well as in industry. Owing to their outstanding electrical and optical properties, CPs have found their use in numerous applications including energy storage,3−6 organic light emitting diodes,7,8 biosensors,9 and photovoltaics,10 as well as actuators.11 It is generally accepted that two different types of charged defects, polarons and bipolarons, are responsible for the electronic conductivity in CPs. Both charge carriers may form upon oxidation (p-doping) or reduction (n-doping) of the neutral polymer. Polarons are generally observed at modest doping levels and carry a spin of 1/2. When additional charges are introduced into the polymer, two polarons may recombine to form a spinless bipolaron.12 The evolution of these two species in CPs has been monitored by various spectrochemical methods13−16 and it has been concluded that either of these two species could be the charge carriers giving rise to the conductivity of CPs, depending on the material.15,16 The bulk conductivity of CPs depends on several parameters, for example, the level of doping, chain and conjugation lengths, and interchain interactions, as well as degree of disorder. Most of these parameters depend on the morphology of the polymer, which is therefore a key factor for controlling the conductivity.17 Different preparation and doping methods18 produce polymer materials with different morphologies ranging from highly disordered amorphous to crystalline materials.19 © 2017 American Chemical Society
The degree of disorder in CPs dictates the way that charge carriers are transported through the material. By tuning the degree of disorder, the materials can undergo a transition from a Fermi-glass with activated hopping-type charge transport to metallic state, with free-electron-like charge transport properties.20,21 Conducting redox polymers (CRPs) consist of a CP backbone decorated with covalently linked redox active pendant groups, and they show promise as electrode materials for secondary batteries.22−25 For application of CRPs as electrode materials the combination of the high charge storage capacity and well-defined redox process of the pendant with the high conductivity of CPs is sought for and it is therefore desirable to preserve the individual properties of the two components. Although several CRPs have been previously reported in the literature,26−28 only few reports on the mechanism of charge transport in these systems exist despite its importance for their applications in electronic devices.29 It is hence largely unknown whether the properties of the CP backbone can be conserved when substituted with redox active pendants. To address this point, we present in this report a mechanistic study of the charge transport in poly-3-((2,5hydroquinone)vinyl)-1H-pyrrole (P1, Scheme 1). The investigation is performed by in situ conductance measurements during electrochemical redox conversion at various temperReceived: April 19, 2017 Published: May 24, 2017 13078
DOI: 10.1021/acs.jpcc.7b03671 J. Phys. Chem. C 2017, 121, 13078−13083
Article
The Journal of Physical Chemistry C
with RE, CE, and a digital thermometer (VWR I620−2000). In order to control the temperature of the cell, an electrochemical cell with a thermostat jacket was connected to a thermostat bath (VWR MX/RL-20), and an ethylene glycol/water (1:3) mixture was circulated by a pump, providing a controlled temperature for the electrochemical cell. During the conductivity measurement, bipotentiostat cyclic voltammetry was performed with a fixed bias potential of 10 mV between the two WEs. The conductance was calculated using the following equation:30
Scheme 1. Chemical Structure of Poly-3-((2,5hydroquinone)vinyl)-1H-pyrrole (P1)
G= atures and doping levels, and by in situ Electron Paramagnetic Resonance (EPR) spectroscopy. We show that the charge transport occurs through a thermally activated polaron− polaron disproportionation/comproportionation reaction and that the polaron is the dominant charge carrier.
Δi 2E bias
(1)
where Δi is the current difference between the two WEs and the Ebias is the bias potential. In Situ EPR Measurements. In situ EPR measurements were performed using a Bruker BioSpin EMX-micro spectrometer equipped with an EMX-Premium bridge and an ER4119HS resonator. An electrolytic flat cell from Wilmad LabGlass was used as a sample holder. P1 was electropolymerized onto a platinum electrode from a MeCN solution of the monomer (10 mM) using cyclic voltammetry. Fifteen scans were used at scan rate of 0.1 V/s between −0.1 and +0.7 V versus Fc0/+. After polymerization, the polymer was washed with acetone, EtOH, and water. During the EPR measurement (microwave frequency 9.74 GHz, microwave power 5 mW, modulation amplitude 1 G, room temperature), potential steps were employed in the potential region between −0.1 and 0.4 V versus SHE with 0.1 V intervals. The material was allowed to equilibrate at the new potential for 100 s before acquiring the spectra.
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EXPERIMENTAL SECTION All reagents were purchased from Sigma-Aldrich and were used without further purification. An Autolab PGSTAT302N potentiostat (Ecochemie, The Netherlands) with a bipotentiostat module was used for all electrochemical measurements. Organic electrolytes were prepared with 100 mM tetra-nbutylammonium hexafluorophosphate (TBAHFP) in dry MeCN solution. Two different aqueous electrolytes were used, one with 1.0 M NaCl, buffered with phosphate, borate, and acetate ions (10 mM each) to pH 2.0, and another with 1 M HNO3 with pH 0.0. An Ag-wire in a 10 mM AgNO3 solution with 100 mM TBAHFP supporting electrolyte (−0.096 V vs Fc0/+), kept in a separate compartment, was used as reference electrode (RE) for organic electrolyte. In aqueous electrolyte Ag/AgCl (3 M NaCl, 0.192 V vs SHE) was used as RE that was immersed directly into the electrolyte solution. Before all measurements, the electrolyte was thoroughly degassed by purging with N2(g) for 10 min and the solution was kept under a N2 atmosphere throughout the measurements. All potentials measured in aqueous solutions are reported versus SHE and all potentials measured in organic solutions are reported versus the ferrocene redox couple (Fc0/+). In the interdigitated array (IDA) measurement, the charge extracted from polymer during doping was evaluated by the integral of the CV during the oxidative scan without any baseline correction. In the in situ EPR measurement, the charge was recorded during the potential step measurement while acquiring the EPR spectra. In Situ Conductance Measurement. IDA electrodes with 90 pairs of interdigitated Au strips with 10 μm width and 150 nm height, separated by 10 μm gaps, and with a circular active area with a diameter of 3.5 mm, purchased from Micrux (Spain), were used as WE and a Pt wire was used as counter electrode. The polymer for in situ conductance measurement was synthesized by electropolymerization onto the IDA WEs from a MeCN solution of the monomer (50 mM) using cyclic voltammetry. To ensure that sufficient polymer contact between the WEs was established, 25 scans were executed at 50 mV/s between −0.1 and +0.7 V versus Fc0/+. After polymerization, the polymer film was washed carefully with acetone, EtOH, and water in order to remove soluble oligomers and unreacted monomers. A four-electrode setup was used for the in situ conductance measurements. A homemade holder that fixated the IDA electrode with the two WEs connected to Au wires was assembled and then immersed into the electrolyte, together
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RESULTS AND DISCUSSION Conductance Measurements. A cyclic voltammogram (CV) of P1, electrochemically polymerized onto a glassy carbon electrode, is shown in Figure 1. It shows the typical
Figure 1. (a) CV of P1 (scan rate at 0.1 V/s); (b) in situ conductance measurement of P1 (scan rate at 1 × 10−3 V/s) in buffered aqueous electrolyte (1 M NaNO3, 10 mM phosphate, borate, and acetate buffer, pH 2.00).
capacitive behavior of the polypyrrole (PPy) backbone with an on-set of polymer doping at around 0.20 V versus the standard hydrogen electrode (SHE) followed by a current plateau at potentials between 0.25 and 0.40 V. Furthermore, the redox reaction of the hydroquinone pendant group is seen as a reversible oxidation/reduction peak-pair with E0′ at 0.45 V versus SHE (the average of the oxidation and reduction peak 13079
DOI: 10.1021/acs.jpcc.7b03671 J. Phys. Chem. C 2017, 121, 13078−13083
Article
The Journal of Physical Chemistry C potentials). As seen from the in situ conductance measurement (curve b in Figure 1) the conductance was very low (
