M.Sc Thesis


M.Sc StudentElad Sharlin
SubjectThermodynamic Stability of Semiconductor Photoelectrodes for
Water Splitting
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Rothschild Avner
Full Thesis textFull thesis text - English Version


Abstract


Semiconductor photoelectrodes for solar-induced water photoelectrolysis as a means to store solar energy in the form of hydrogen must have a unique combination of optical, transport and electrochemical properties in order to achieve high solar to hydrogen conversion efficiency. In addition, they should also comprise of earth abundant elements and be stable in aqueous solutions. The lion’s share of semiconductor materials that have been examined as candidate photoelectrodes for water splitting were found to be unstable under conditions of water photo-oxidation (photoanodes) or photo-reduction (photocathodes). Thus, stability is one of the most critical challenges in the development of photoelectrodes for solar energy conversion and storage.

In this work we examine the thermodynamic stability of semiconductor photoelectrodes for water splitting. We revisit the potential - pH equilibrium diagrams introduced by Marcel Pourbaix to describe the corrosion of metals in aqueous solutions, and Heinz Gerischer’s phenomenological model of photodecomposition of semiconductor electrodes in contact with aqueous solutions. By marrying these two seminal legacies we formulate a new methodology for understanding and evaluating the thermodynamic stability of semiconductor photoelectrodes by using quasi Pourbaix diagrams that take into account the effect of light on the Nernst potential of electrochemical reactions occurring at the semiconductor electrode / aqueous electrolyte interface. We demonstrate this analysis approach in details using Cu2O photoelectrodes as a case study, and expand it to complex material compositions, mainly ternary oxides (SrTiO3 and SrFeO3) and oxynitrides (TaON), using the methodology developed by Harumi Yokokawa to construct phase diagrams in the chemical potential space.

This work provides a solid background for understanding and evaluating the thermodynamic stability of semiconductor photoelectrode in aqueous solutions, consolidating the use of phase diagrams in this field. The beauty of phase diagrams is that they contain the complexity of multiple electrochemical reactions with multi parameters and present them in a graphical diagram that is easy to understand by direct visual examination. This approach, we believe, will significantly advance the research and development of semiconductor photoelectrodes for solar energy conversion and storage by providing an easy tool to assess the thermodynamic stability of candidate materials, saving a lot of futile efforts in investigating unstable compositions.