In electrochemical systems, metal surface charging phenomena dictate the strength of electrostatic interactions between the electrified electrode and ions in solution. These effects are of vital importance for electrochemical reactions in general. Historically, the potential of zero charge (pzc) of the metal has been employed to parametrize the surface charging relation. The structural model of the electrified interface presented in this article goes beyond the oversimplified pzc concept by accounting for the formation of surface oxide and the orientational ordering of interfacial water molecules. The analytical solution of the model reveals a peculiar non-monotonic charging behavior. The Pt surface exhibits a negative effective charge in a low potential region, a positive charge in an intermediate potential region, and a negative charge in a high potential region due to surface oxide dipoles. This non-monotonic behavior is in agreement with a seminal experimental work of Frumkin and Petrii [Electrochim. Acta197520347359] that had remained hitherto unexplained.

Electrochemical impedance spectroscopy is a convenient and powerful tool in characterizing porous electrodes in electrochemical systems. The full potential of impedance characterization can only be achieved when a physically meaningful impedance model is used. This study aims to build a theoretically-consistent framework and to develop a series of impedance models for porous electrodes with different properties. The framework starts from a full problem formulated with the concentrated solution theory and then simplifies it to four limiting cases. In-plane, through-plane, multi-dimensional inhomogeneities are considered. In addition, based on the analytical Fourier transform, the impedance response during dynamic process is numerically calculated. The models developed in this study are then applied to three typical cases: blocking electrode, electrode with faradaic reactions, and electrode constituted of particles with insertion reactions, corresponding to their practical counterparts in electrochemical capacitors, polymer electrolyte fuel cells, and lithium-ion batteries, respectively. In each case, the structure, asymptotic behaviors, and characteristic frequencies of and inhomogeneous effects on the impedance spectrum are analyzed. Special attention is paid to the assumptions and applicability of each model. Experimental strategies to justify the use of an electric circuit model are discussed. The use of Warburg impedance to fit the diffusion coefficient in the solid active particles is scrutinized.

Proton Exchange membrane fuel cells (PEMFCs) for automotive applications are subject to hundreds of thousands of potential or load cycles during normal operation of the vehicle. Pt dissolves under such cycling and is a major contributing factor to the lowered durability of PEMFCs. This paper details the effect of various simplified potential-cycle profiles, the effect of operating conditions such as temperature, relative humidity, reactant gases etc., as well as different catalyst materials under normal fuel cell operating voltage ranges (0.95-0.60V) in fuel cells. Results on Pt dissolution utilizing complementary ex-situ methods are also presented. The fundamental and practical implications of varying the operating parameters as well as a possible hypothesis for Pt dissolution are reported.