Electrode materials computer simulation

In the next section a brief layout of simulation methods will be given. Then, some basic properties of the models used in computer simulations of electrochemical interfaces on the molecular level will be discussed. In the following three large sections, the vast body of simulation results will be reviewed structure and dynamics of the water/metal interface, structure and dynamics of the electrolyte solution/metal interface, and microscopic models for electrode reactions will be analyzed on the basis of examples taken mostly from my own work. A brief account of work on the adsorption of organic molecules at interfaces and of liquid/liquid interfaces complements the material. In the final section, a brief summary together with perspectives on future work will be given. 

The computer simulations described in this chapter are very simplistic models, with parameters obtained by fitting to actual stimulation waveforms. This type of simulation can only reflect the electrode behavior within a narrow range of charge-injection settings. Nevertheless, with better understanding of the materials, future models might be able to predict the electrode behavior at all charge-injection conditions, and even predict the rate of corrosion. 

For a given pair of electrode reactions of known thermodynamic and kinetic characteristics, electrochemical engineering procedures must provide a reactor design in which these reactions can occur with high material and energy efficiencies. Simultaneously, appropriate provisions have to be made for the input of reactants and outflow of products and for the addition (or removal) of electric and thermal energy. The emphasis here is on the complete system and the inter-related surface reactions and transport processes. System analysis and design of electrochemical reactors require elaborate computer-implemented process simulation, synthesis, and optimization. 

Analyses of responses aiming to define the characteristics of the electrode deposit and the details of the mechanism operative in anodic oxidations or cathodic reductions are also very challenging and rare in the case of amperometric sensing on modified electrodes, due to the complexity of most systems with respect to the bare ones. On the other hand, the superior performance of new computers may lead to unprecedented sophisticated simulations. In the case of composites based on nanosized materials, studies of the diffusion to micro- and nano-electrode systems may be exploited [234]. Similarly, calculations regarding the reaction mechanisms based on density functional theory have been exploited to give a rationale to the performance of a number of electrocatalysts [235]. 

Now that supercapacitors are better understood, the next objective for simulations consists in proposing optimized electrode - electrolyte combinations or new storage concepts. To this end, high-throughput screening seems like a promising way. Indeed, in the spirit of what is currently done in materials science for photocatalysts " and for battery electrodes or electrolytes. First steps towards this direction have been made by Balducci et in order to predict via computation and to test experimentally the best electrolytes for supercapacitors. 

It is almost impossible to cover the entire range of models in Figure 25.1, and in this chapter we will limit ourselves to the different modeling approaches at the continuum level (micro-macroscopic and system-level simulations). In summary, there are computational models that are developed primarily for the lower-length scales (atomistic and mesoscopic) which do not scale to the system-level. The existing models at the macroscopic or system-level are primarily based on electrical circuit models or simple lD/pseudo-2D models [17-24]. The ID models are limited in their ability to capture spatial variations in permeability or conductivity or to handle the multidimensional structure of recent electrode and solid electrolyte materials. There have been some recent extensions to 2D [29-31], and this is still an active area of development As mentioned in a recent Materials Research Society (MRS) bulletin [6], errors arising from over-simplified macroscopic models are corrected for when the parameters in the model are fitted to real experimental data, and these models have to be improved if they are to be integrated with atomistic... 

The interface between a solid electrode and a liquid electrolyte is a complicated many-particle system, in which the electrode ions and electrons interact with solute ions and solvent ions or molecules through several chatmels of interaction, including forces due to quantum-mechanical exchange, electrostatics, hydrodynamics, and elastic deformation of the substrate. Over the last few decades, surface electrochemistry has been revolutionized by new techniques that enable atomic-scale observation and manipulation of solid-liquid interfaces, yielding novel methods for materials analysis, synthesis, and modification. This development has been paralleled by equally revolutionary developments in computer hardware and algorithms that by now enable simulations with millions of individual particles, so there is now significant overlap between system sizes that can be treated computationally and experimentally. 

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