Interface resolved simulations of continuum scale electrochemical hydrogen evolution

Abstract

An important aspect of improving the efficiency of water electrolysis is to remove the electrochemically generated hydrogen and oxygen bubbles. The evolution of these gases, which are associated with increased electrical resistance, are driven by electrochemical reactions causing supersaturation of the electrolyte which leads to bubble nucleation, growth, and eventual detachment from the electrode. Due to the different physics as well as the length and time scales associated with the process, referred to as the multiscale and multiphysics nature, predicting the bubble evolution using analytical models is challenging. As numerical modelling approaches, like Computational Fluid Dynamics (CFD), predicts the fluid flow based on the underlying governing equations, it can be used to study electrochemical bubble evolution. The work undertaken during the PhD is primarily to develop and verify a multiphysics CFD framework based on the Volume of Fluid (VOF) method available in OpenFOAM® for continuum scale hydrogen bubbles. In the context of this work, continuum scale bubbles refers to bubble diameters which are larger than a few hundred micrometers. The VOF method is customized by adding the physics and numerical techniques relevant to treating electrochemical reactions, dissolved gas transport, charge transport, interfacial mass transfer and associated bubble growth (from a pre-existing submillimeter bubble). The proposed framework is developed incrementally, with each step corresponding to implementation and verification of a multiphysics module, eventually culminating in the fully coupled multiphysics framework. This modularized approach allows for verification of the implemented functionality with existing theoretical models and/or computational benchmarks. The thesis, in essence, provides context to the undertaken research, review of the various modelling techniques used to treat the multiphysics nature of electrochemical hydrogen evolution and details of developed framework. In addition, the thesis also summarizes knowledge gained during the PhD about the solution procedure used in OpenFOAM® and the VOF method to enable knowledge dissemination for further research