Proton conductor-based solid oxide fuel cells (SOFCs) and electrolysis cells (SOECs) are receiving increasing attention because of their potential for operation at intermediate temperatures (400 - 600 ^oC) with high energy efficiency at low cost. In addition, water is formed/provided on the air electrode side of proton-conducting cells, effectively avoiding fuel dilution and nickel oxidation problems associated with oxide-ion conductor-based cells. To date, doped barium cerates-based perovskite oxides are the most widely adopted proton conducting electrolytes due to their desired electrochemical properties. To achieve high proton conductivity, acceptor doping with rare earth elements is a commonly used strategy, which is critical to the formation of protonic defects. Although many trivalent elements have been studied as dopants in the barium cerate family and reasonable electrochemical performance has been demonstrated, the effect of acceptor dopants on other properties of electrolyte materials, especially in single cells under operating conditions, is yet to be studied in detail. In this presentation, we will report our recent progress in the development of a series of acceptor-doped proton-conducting electrolytes. The results reveal that conductivity, transference number, chemical stability, and compatibility with NiO are all closely correlated with dopant size. In particular, the reactivity with NiO is found to strongly affect the properties of the electrolytes and hence cell performance. Among all tested compositions, an optimized electrolyte shows excellent chemical stability and minimal reactivity towards NiO, as predicted from density functional theory (DFT)-based calculations and confirmed by experimental results. In addition, reversible protonic ceramic electrochemical cells (R-PCECs) based on the optimized electrolyte demonstrate exceptional performance and stability, achieving a remarkable peak power density of 1.2 W cm^-2 at 600 ^oC in the fuel cell mode and a high current density of 2.0 A cm^-2 at 1.3 V and 600 ^oC in the steam electrolysis mode while maintaining long-term durability for over 1000 h without obvious degradation.