Quantifying Electronic and Ionic Conductivity Contributions in Carbon/Polyelectrolyte Composite Thin Films

Abstract

A method for independently obtaining electronic and ionic contributions to electrical conductivity in thin-film samples comprised of carbon black (CB) mixed with polymer electrolyte is reported. The method relies upon careful control of the nature of the contact between the sample and the current-carrying electrodes. Electronic conductivities are derived from currents obtained using electronically-conductive glassy carbon electrodes to contact the sample, whereas ionic conductivities are derived from currents obtained using ionically-conductive Nafion electrodes to contact the sample. Conditions under which the measured currents are free from interference from redox reactions at the electrodes and capacitive charging at electrode-electrolyte interfaces are discussed. Electrical conduction was studied in a series of composite carbon black/polymer samples under conditions of fixed temperature and variable relative humidity (RH). For samples containing 10-20 weight percent carbon black dispersed in Nafion, electronic conductivity was higher than ionic conductivity at all RH values tested (25-100% RH). Ionic conductivity increased with increasing RH in a manner consistent with expectation from prior studies on Nafion membranes, whereas electronic conductivity decreased with increasing RH due to water swelling and weakened electrical contacts among carbon particles in the network. Electrical conduction in materials may arise from motion in an electric field of electrons, ions, or both. For many materials the charge carrier identity is implicit; e.g. for metals and semimetals such as carbon, charge is understood to be carried by mobile electrons, whereas for electrolyte materials such as salts dissolved in solvents and most polymer electrolytes, charge is understood to be carried by mobile ions.1,2 Some materials have the very interesting property that they may transport charge via both ions and electrons; such materials are referred to as mixed ionic electronic conductors (MIECs).3-5 MIEC materials are critically important in many electrochemical technologies, for example in battery electrodes, 6-13 fuel-cell electrodes, [14][15][16] and in certain types of membrane reactor. 17-20A simple measurement of electrical resistance/conductance on a bulk sample, for example from the current obtained upon application of a potential difference between two points on the sample using current-carrying electrodes, can provide values for resistivity/conductivity but it does not provide information on the nature of the charge carrier(s). Four-point-probe (e.g. van der Pauw) measurements are useful for eliminating the influence of contact resistance at the current-carrying electrodes on the measured resistance but such measurements still do not distinguish whether charge is carried by electrons, ions, or both. For some special situations, e.g. doped semiconductors, information about charge-carrier identity may be obtained from the behavior of the sample in response to an external perturbation. For example, in the case of Hall-effect ...

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“…2018.23.47019.17324 Транспортные свойства топливных элементов являются одним из важных факторов, определяющих их эффективность [1]. Зарядовый транспорт в электродах этих структур, содержащих протонпроводящий полимер типа Nafion и металлоуглеродный компонент (Pt/C), обычно лимитирован транспортом протонов, так как ионная проводимость Nafion на несколько порядков меньше электронной проводимости углеродной сажи [2]. Поскольку Nafion является не единственным компонентом электрода и увеличение его доли влечет уменьшение пористости и площади доступной поверхности платины [3], одной из задач при разработке электрода является оптимизация его компонентного состава, в частности содержания Nafion, с тем, чтобы была обеспечена хорошая газопроницаемость (массовый транспорт) при относительно небольшом ионном сопротивлении.…”