# Electrochemistry¶

Electrochemistry in CatMAP is implemented as a set of thermodynamic corrections in the enthalpy_entropy module. To run models containing electrochemical reactions in CatMAP, the user needs to provide the following in addition to all of the typical components of a CatMAP model described in the tutorials:

• Some sort of voltage-dependent species. Currently, pe and ele are supported as fictitious gas molecules that represents the free energy of a proton-electron pair at 0V vs RHE or an electron at 0V vs SHE, respectively. As part of the Computational Hydrogen Electrode, the pe gas species should have the same free energy of 1/2 of a molecule of H2 gas. This species’s energy (usually 0 if an H2 reference is used) can be defined manually in the input file, but defaults to half the free energy of the H2 gas molecule in your system (if using pe) or 0 if using “ele”. These are special electrochemistry-specific gas-phase species that should ignore your choice of free energy correction scheme.
• In the reaction definitions, any potential-dependent reaction should have the pe_g gas molecule or ele_g in the initial (if reductive) or final (if oxidative) state. In addition, in the reaction definitions and input file, transition states for these reactions should contain pe or ele in the species name. For example, the reduction of adsorbed oxygen to adsorbed OH can be written as 'O* + pe_g <-> O-pe* -> OH*'.
• An alternative formulation for electrochemical transition states is available via a special notation shown in the ORR tutorials. Instead of explicitly defining a transition state species in your input file and using it in a reaction expression, you may instead write something like ^0.26eV_a where the 0.26eV represents the free energy barrier of elementary step at that step’s equilibrium/limiting potential on site a.
• A voltage (vs RHE) defined as a parameter in the .mkm file e.g. voltage = -0.2. This is also a global thermodynamic variable, and can be manipulated as a descriptor as shown in the ORR_thermo tutorial.
• A transfer coefficient defined as a parameter in the .mkm file e.g. beta = 0.5. This value can be defined for each elementary step through a reaction-specific flag like ;prefactor=None, beta=0.45 at the end of the corresponding reaction expression string. See the ORR tutorials for an example of this notation.
• In the .mkm file, electrochemical_thermo_mode can be specified to one of three possible values (defaults to simple_electrochemical): simple_electrochemical, hbond_electrochemical, and hbond_with_estimates_electrochemical. “simple” only adds free energy corrections to adsorbates and transition states to account for voltage and beta. “hbond” will take a default or user-provided hbond_dict to correct each species for hydrogen bonding stabilization (see catmap/data/hbond_data.py for the default hbond_dict). “hbond_with_estimates” attempts to estimate a hydrogen bonding correction for a given species based on its chemical formula. This is a very crude process that is described in catmap/thermodynamics/enthalpy_entropy.py.

There are a few provided examples of using electrochemistry in CatMAP in the tutorials/electrochemistry directory. The HER example shows the hydrogen evolution reaction in the low-coverage limit, and the provided README file explains the details of the simplest of electrochemical reactions as done in CatMAP. The ORR example is meant to reproduce the results discussed in Hansen et al. (2014) DOI: 10.1021/jp4100608, which used a home-spun microkinetic model. The README in that directory goes into depth on how you can replicate some of the major features of this work with the provided scripts. The electron example shows how to equivalently use CatMAP for an SHE potential versus an RHE one.