This paper analyzes the small signal ac impedance of electron diffusion and recombination in a spatially restricted situation with application in systems such as porous TiO2 nanostructured photoelectrodes and intrinsically conducting polymers. It is shown that the diffusion−recombination model with the main types of boundary conditions assumes a finite set of possible behaviors in the frequency domain, which are classified according to relevant physical parameters. There are four possible cases:  (i) the impedance of finite diffusion with reflecting boundary, (ii) the impedance of finite diffusion with absorbing boundary, (iii) the impedance of diffusion-reaction in semiinfinite space or Gerischer impedance, and (iv) the impedance that combines Warburg response at high frequency and a reaction arc at low frequency. The generality of the approach is discussed in terms of the spatial distribution of the electrochemical potential or quasi-Fermi level and also in terms of the transmission line representation....

Theory of the Impedance of Electron Diffusion and Recombination in a Thin Layer

This paper presents a review of the redox and electrocatalytic properties of transition metal oxide electrodes, paying particular attention to the oxygen evolution reaction. Metal oxide materials may be prepared using a variety of methods, resulting in a diverse range of redox and electrocatalytic properties. Here we describe the most common synthetic routes and the important factors relevant to their preparation. The redox and electrocatalytic properties of the resulting oxide layers are ascribed to the presence of extended networks of hydrated surface bound oxymetal complexes termed surfaquo groups. This interpretation presents a possible unifying concept in water oxidation catalysis – bridging the fields of heterogeneous electrocatalysis and homogeneous molecular catalysis.

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Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes

Processes in the dark of electron transport and recombination in several nanoporous titanium dioxide films have been studied as a function of the applied potential, using the electrochemical impedance technique. Contact and bulk characteristics have been identified, decoupled, and interpreted, applying a transmission line model that identifies the following elements:  (i) the capacitance of the interface between the exposed surface of the substrate and the electrolyte, (ii) the electron transport resistance, (iii) the charge-transfer resistance distributed in the TiO2/electrolyte interface, and (iv) a distributed capacitive element related to charging the porous matrix. The model provides a satisfactory description of the spectra in widely different conditions of conductivity of the TiO2 phase. The electron conductivity has been determined as a function of applied potential and coincides for the different samples under study. Classical electrochemical frameworks of transport and interfacial charge transfe...

Decoupling of Transport, Charge Storage, and Interfacial Charge Transfer in the Nanocrystalline TiO2/Electrolyte System by Impedance Methods

A high-resolution impedance study of the hydrogen oxidation in Ni/8YSZ (yttria-stabilized zirconia) cermet anodes has been realized in consideration of a broad range of operating conditions (temperature and partial pressure of fuel gas components H 2 , H 2 O, N 2 , He). A major problem in this respect concerns the origin and physical interpretation of empirical equivalent circuits used to fit the experimental data. We applied a two-stage approach for the evaluation of the impedance data: (i) at first, by the deconvolution of a distribution function of relaxation times (DRT), four different processes and their characteristic relaxation times have been identified. Two processes at frequencies < 1 kHz represent a gas-conversion process or, respectively, a gas diffusion, whereas two processes at higher frequencies (2-30 kHz) are associated with the electro-oxidation of hydrogen at active sites, including the charge transfer reaction and the ionic transport. (ii) Subsequently, the last mentioned processes were fitted to a "transmission line" model describing the electronic and ionic transport properties of the Ni/8YSZ cermet. The high resolution of the DRT combined with the numeric accuracy of the complex nonlinear least square (CNLS) fit enabled us to determine (i) the effective ionic conductivity of the Ni/8YSZ cermet, (ii) the spatial extension of the electrochemically active area adjacent to the electrolyte/electrode interface, and (iii) the charge transfer resistance and its thermal activation energy.

Combined Deconvolution and CNLS Fitting Approach Applied on the Impedance Response of Technical Ni ∕ 8YSZ Cermet Electrodes

Biomimetic electrochemistry from conducting polymers. A review: Artificial muscles, smart membranes, smart drug delivery and computer/neuron interfaces

Germà García Belmonte

Papers

Application of a distributed impedance model in the analysis of conducting polymer films