Towards improved stability in proton-conducting ceramic fuel cells

Stability of proton-conducting ceramic fuel cells (PCFCs) is not well understood. Many stable cells are reported in the literature. However, unstable cells generally go unstudied. This paper explores and identifies sources of instability, and suggests fabrication methods that promote cell stability. We find that fuel cells that lack gas-tight electrolytes breakdown over time at rates exceeding 75% of the cell overpotential per 1000 h. Oxygen leakage through electrolyte pinholes gives rise to nickel re-oxidation at the negatrode, advancing performance degradation. Nickel redox cycling decreases the catalytic activity and leads to mechanical cleaving of the negatrode. The nickel oxidation increases high-frequency polarization resistance which is evident in the distribution of relaxation times (DRT). Nickel separation and cleaving from the electrolyte phase within the cermet negatrode increases ohmic resistance. Three sintering strategies aid the formation of gas-tight electrolytes, including two-step sintering, sintering neighbors, and positrode functional layers (PFLs). Stability rates of gas-tight cells are as low as 2.6% of the overpotential per 1000 h. Fabricating cells with thicker electrolytes also improves stability. Hybrid-DRT polarization mapping during stability testing reveals that PCFCs have four main DRT peaks, and thus four main electrochemical processes, at 550 degrees C.