Abstract Proton-conducting solid electrolytes composed of gadolinium-doped barium cerate (BCG) or gadolinium and praseodymium-doped barium cerate (BCGP) were tested in an intermediate-temperature fuel cell in which hydrogen or ammonia was directly fed. At 700 °C, BCG electrolytes with porous platinum electrodes showed essentially no loss in performance in pure hydrogen. Under direct ammonia at 700 °C, power densities were only slightly lower compared to pure hydrogen feed, yielding an optimal value of 25 mW cm −2 at a current density of 50 mA cm −2 . This marginal difference can be attributed to a lower partial pressure of hydrogen caused by the production of nitrogen when ammonia is decomposed at the anode. A comparative test using BCGP electrolyte showed that the doubly doped barium cerate electrolyte performed better than BCG electrolyte. Overall fuel cell performance characteristics were enhanced by approximately 40% under either hydrogen or ammonia fuels using BCGP electrolyte. At 700 °C using direct ammonia feed, power density reached 35 mW cm −2 at a current density of approximately 75 mA cm −2 . Minimal loss of performance was noted over approximately 100 h on-stream in alternating hydrogen/ammonia fuels.