Poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is the most studied and explored mixed ion‐electron conducting polymer system. PEDOT:PSS is commonly included as an electroactive conductor in various organic devices, e.g., supercapacitors, displays, transistors, and energy‐converters. In spite of its long‐term use as a material for storage and transport of charges, the fundamentals of its bulk capacitance remain poorly understood. Generally, charge storage in supercapacitors is due to formation of electrical double layers or redox reactions, and it is widely accepted that PEDOT:PSS belongs to the latter category. Herein, experimental evidence and theoretical modeling results are reported that significantly depart from this commonly accepted picture. By applying a two‐phase, 2D modeling approach it is demonstrated that the major contribution to the capacitance of the two‐phase PEDOT:PSS originates from electrical double layers formed along the interfaces between nanoscaled PEDOT‐rich and PSS‐rich interconnected grains that comprises two phases of the bulk of PEDOT:PSS. This new insight paves a way for designing materials and devices, based on mixed ion‐electron conductors, with improved performance.

Ladder-type “torsion-free” conducting polymers (e.g., polybenzimidazobenzophenanthroline (BBL)) can outperform “structurally distorted” donor–acceptor polymers (e.g., P(NDI2OD-T2)), in terms of conductivity and thermoelectric power factor. The polaron delocalization length is larger in BBL than in P(NDI2OD-T2), resulting in a higher measured polaron mobility. Structure–function relationships are drawn, setting material-design guidelines for the next generation of conducting thermoelectric polymers

have been reported in inorganic ionic solids [ 15 ] and electrolytes, [ 16,17 ] where ions are the only charge carriers. But to the best of our knowledge, the ionic thermoelectric effect in conducting polymers has not been studied and reported before.Here, we investigate the role of ions in the thermoelectric response of different PEDOT derivatives. We observe surprisingly large increases in the thermo-induced voltage at high humidity levels, of up to several hundreds of µV/K, which is identifi ed as an ionic Seebeck effect. The ionic thermovoltage and its potential for improving the thermoelectric effi ciency in conducting polymers are discussed.Five different PEDOT derivatives of different electrical conductivities (measured at 10% RH, 300 K) are deposited on glass substrates including two gold electrode patterns separated by 1 mm: (i) PEDOT-Tos (thickness = 627 nm, σ = 15 200 S m −1 ) synthesized by using solution oxidative in situ polymerization, [ 18 ] (ii) PEDOT-PSS-DEG (4.75 µm, σ = 530 S m −1 ) obtained by the addition of 2 wt% of the secondary dopant diethylene glycol (DEG) [ 19 ] into the PEDOT-PSS dispersion, (iii) PEDOT-PSS (5.68 µm, σ = 14 S m −1 ) as the commercial water dispersion called Baytron P by H. C. Starck, (iv) PEDOT-PSSPSSNa (3.43 µm, σ = 0.08 S m −1 ) obtained by the addition of 2 wt% of PSSNa to the PEDOT-PSS dispersion, and (v) selfdoped PEDOT-S (116 nm, σ = 75 S m −1 ), in which the sulfonate dopant groups are covalently linked to the PEDOT chains. PEDOT-S has been synthesized in the lab. [ 20 ] PEDOT-Tos exhibits the highest electrical conductivity. PEDOT-Tos possesses a high density of conducting PEDOT chains (EDOT/sulfonate molar ratio is 2.7) packed in a paracrystalline structure. [ 18 ] In PEDOT-Tos, the tosylate moieties are the anions and balance the positive charges carried along the conducting polymer chains. The included ions are effectively immobile in the polymer; it is an electronic (hole) conductor. In PEDOT-S, the EDOT/sulfonate ratio is 1.

The thermoelectric (TE) phenomena are intensively explored by the scientific community due to the rather inefficient way energy resources are used with a large fraction of energy wasted in the form of heat. Among various materials, mixed ion-electron conductors (MIEC) are recently being explored as potential thermoelectrics, primarily due to their low thermal conductivity. The combination of electronic and ionic charge carriers in those inorganic or organic materials leads to complex evolution of the thermovoltage (Voc) with time, temperature and/or humidity. One of the most promising organic thermoelectric materials, poly(3,4-ethyelenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS), is a MIEC. A previous study reveals that at high humidity, PEDOT-PSS undergoes an ionic Seebeck effect due to mobile protons. Yet, this phenomenon is not well understood. In this work, we study the time dependence of the Voc and explain its behavior from the contribution of both charge carriers (holes and protons). We identify the presence of a complex reorganization of the charge carriers promoting an internal electrochemical reaction within the polymer film. Interestingly, we demonstrate that the time dependence behavior of Voc is a way to distinguish between three classes of polymeric materials: electronic conductor, ionic conductor and mixed ionic-electronic conductor.

SignificanceSpreading electrochemical technologies, such as energy, bioelectrochemical devices, and industrial electrochemical synthesis, require low-cost large area electrodes. Conducting polymers possess a unique combination of properties compared with most of the inorganic electrodes: acid resistance, the absence of surface-insulating oxide, low temperature and solution processability, a high natural abundance of their elements, molecular porosity. Conducting polymers are inhomogeneous conductors composed of ordered and disordered regions through which electronic transport takes place via percolation paths. We discovered that the density of percolation paths in the bulk of the material dictates the rate of electron transfer at the electrolyte–polymer electrode interface. This reveals one of the key parameters of designs to achieve efficient electrochemical technologies based on polymer electrodes.