In spite of various advantages of protonic ceramic fuel cells over conventional fuel cells, distinct scepticism currently remains about their applicability because of lower-than-predicted performance and difficulty with scale-up. These challenges mainly stem from the refractory nature of proton-conducting ceramic electrolytes and the low chemical stability of these materials during the sintering process. Here, we present the fabrication of a physically thin, structurally dense and chemically homogeneous electrolyte, BaCe0.55Zr0.3Y0.15O3-δ (BCZY3), through a facile anode-assisted densification of the electrolyte on a structurally and compositionally uniform anode support, which resulted in breakthroughs in performance and scalability. A BCZY3-based protonic ceramic fuel cell with a size of 5 × 5 cm2 exhibits an area-specific ohmic resistance of 0.09 Ω cm2 and delivers a power as high as 20.8 W per single cell at 600 °C. Protonic ceramic fuel cells (PCFCs) operate at lower temperatures than solid oxide fuel cells but suffer from lower performances, especially during scale-up. Here, the authors report a 25 cm2 PCFC based on a BaCe0.55Zr0.3Y0.15O3–δ electrolyte that displays a record-high power density of 20.8 W at 600 °C.