Extended Data Figure 6: H-SNO fuel cell performance.
a, The dependence of micro-fabricated SOFC performance on the thickness of the SNO electrolyte at 500 °C. We fabricated a series of samples with various thicknesses of the electrolyte while keeping identical deposition conditions for the cathode and anode. By doing so, the electrolyte Ohmic resistance is varied while the electrode polarization resistance is kept more or less a constant. A clear increase in OCV with increasing thickness can be seen, which could be due to the decrease in the electrode polarization loss because of the larger electrolyte Ohmic resistance, as discussed in Extended Data Fig. 5. The power density does not show much dependence on the electrolyte thickness, because thicker electrolytes leads to higher Ohmic resistance, but also higher OCV. b, Performance of Pt/SNO/Pd micro-fabricated SOFCs with a dense Pd anode with 3% humidified pure H2 as fuel and laboratory air as oxidant. It has been shown that hydrogen primarily creates protonic defects rather than oxygen vacancies in SNO (ref. 12). To verify that protons are the dominant mobile ion species in SNO and H-SNO, we fabricated an SOFC with the SNO electrolyte, a dense 100-nm-thick Pd anode, and a porous 100-nm-thick Pt cathode. Pd anode is known as a protonic conductor but an oxygen ion barrier and can therefore filter out any oxygen ion transport. This verifies that protons rather than oxygen ions are the dominant mobile ions in SNO. During the fuel cell testing, 100 sccm pure H2 was flowed on the anode side, with the cathode exposed to air. The fuel cell with dense Pd has an OCV of 0.6 V and a peak power density of 24 mW cm−2 at 500 °C. The protonic conductivity of H-SNO can be extrapolated from impedance spectroscopy and OCV measurements. The similar values of the measured ionic conductivity in cells with Pt and Pd anode confirm that protonic conduction is the dominant ionic transport mechanism.
