Triple-Phase Boundaries (TPBs) in Fuel Cells and Electrolyzers

Abstract

Fuel cells (FCs) and electrolyzers (ELs) have attracted extensive attention for energy conversion and storage due to their high energy efficiency and environmental sustainability. For a solid electrolyte-based system, electrode reactions (such as oxygen reduction, hydrogen oxidation, oxygen evolution, and hydrogen evolution) take place at or near the triple-phase boundaries (TPBs) of the electrodes, or the boundaries among an electronically conducting phase (the electrode), an ionically conducting phase (the electrolyte), and the gas molecules that participate in electrode reactions. While modifying the composition, structure, and morphology or extending the length of TPB can significantly enhance the electrocatalytic activity and stability of the electrodes of FCs and ELs, as demonstrated by experimental and theoretical simulation results, the mechanisms of the performance enhancements are yet to be fully understood. It is still unclear how the microscopic features of the junctions between different phases may impact the rate of charge and mass transfer processes associated with electrode reactions. In this review, we will start with a brief introduction to the basic concept of TPB and how TPB may affect the performance (electrocatalytic activity and durability) of FCs and ELs. Then, we will summarize the latest developments in enhancing the performance of FCs and ELs (especially those based on solid oxide electrolytes) through modification or optimization of TPBs, including altering TPB geometry or morphology, tuning TPB composition, and increasing TPB length of fuel electrodes and air electrodes. Subsequently, we will highlight the unique characteristics of TPBs that are critical to enhancing electrode performance and provide important insights into the mechanisms of performance (activity and durability) enhancement, aiming at achieving rational design of better TPB or electrode materials and architecture. Finally, we will discuss the remaining challenges for knowledge-based design of highly efficient electrodes for electrochemical devices, together with possible directions and future perspectives for development of highly efficient electrodes through optimization of TPBs.