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2.3. THEORETICALELECTROCHEMISTRY 33 reactant, but also of the electrolyte. To make matters even more complex, the elec- trolyte cannot be assumed to be a static conformation of molecules, but a proper description should sample many different configurations of molecules and the en- ergetics for reactions under all such conditions. One way of attacking this problem is by kinetic Monte Carlo methods or (ab initio) molecular dynamics[184]. Not only is the system itself more complicated, but the reaction is controlled not only by temperature and pressure but also by the electrochemical potential. This means that a number of different phenomena also need to be included in a model. These phenomena include the polarization of the electrodes, the charge transfer between the surface of the electrode and the reactants and also the electric field formed between the two polarized electrodes. If the polarization of the electrodes is to be treated rigorously, a complete model should thus include not only a sur- face slab for a single electrode, but rather a system containing both a working electrode, a counter electrode and the electrolyte[152]. Upon applying a bias, the effect of the electric field in the electrolyte between the electrodes, and thus also the charged double layers at the electrodes, could be simulated self-consistently. However, this would require a quite large simulation cell, beyond what is practical today. Furthermore, the non-equilibrium situation with two electrodes each having a different potential (and thus different Fermi levels) is not possible to study with conventional ab initio methods[152]. Not only is the size of the system a chal- lenge, but a method of describing the polarization and charge transfer efficiently is still lacking[184]. Several methods of modeling the polarization of the elec- trode have been put forward, but it is not clear which offers the best compromise between accuracy and cost. Going back to the charge transfer process, Marcus theory is able to describe some details of outer sphere charge transfer[185], but its suitability for describing electrochemical bond formation or bond breaking has been questioned.[184] To add to all these challenges, the actual structure, under polarization, of both the electrolyte and the surface is in general not well known experimentally (although methods are beginning to become available [184, 186]), as the systems are more challenging to study experimentally as well. 2.3.1 The computational hydrogen electrode method The method that has been applied to study electrocatalytic reactions in the present work is the computational hydrogen electrode (CHE) method of Nørskov et al. [152]. It is not the first attempt to model electrochemical reactions at the atomic level, but it is one of the simpler methods and, more importantly, has been shown to have predictive power regarding electrocatalyst activities. It makes significant ap- proximations for the electrochemical reaction and the electrolyte, but has still been found to yield acceptable results that enable understanding of trends in electrocat- alytic activity. The method is based on periodic calculations, and thus allows forPDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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