Improving the performance of protonic ceramic fuel cells and electrolyzers: The role of acceptor impurities in oxide membranes

Abstract

Protonic ceramic electrochemical cells (PCECs) are promising energy conversion devices offering an efficient and environmentally friendly way to produce electricity and pure hydrogen. An important challenge in the development of PCECs is the enhancement of their performance through selection of optimal dopants for acceptor-doped oxide perovskites, commonly used as electrolyte membranes. In this work, we elucidate the implications of the interaction of protons and oxygen vacancies with acceptor impurities in oxide membranes for the performance of PCECs operating in the fuel cell and electrolysis modes. The analysis relies on our recent theoretical developments on fuel cell operation and oxide hydration, as well as on the proposed statistical theory of proton hopping conduction in acceptor-doped oxides. It is shown that the interaction between ionic defects and impurities can substantially affect the characteristics of proton-conducting membranes. General relationships between the output characteristics of PCECs based on membranes with proton and hole conductivity and the energies of acceptor-bound states of ionic defects are determined. It is established that trapping of protons and oxygen vacancies reduces both proton and hole currents and effective conductivities, thereby altering the dependences of the voltage and power density on current density. We demonstrate the possibility of tuning the power density and faradaic efficiency of PCECs by doping oxide membranes using acceptor impurities with varying defect trapping energies. The presented results contribute to understanding the functioning mechanisms of protonic ceramic fuel cells and electrolyzers, and provide new fundamental criteria for the selection of acceptor impurities for oxide membranes to develop high-efficient devices.