Solid oxide fuel cells (SOFC) were fabricated with gadolinia-doped ceria (GDC)-yttria stabilized zirconia (YSZ), thin bi-layer electrolytes supported on Ni+YSZ anodes. The GDC and YSZ layer thicknesses were 45 mu m, and similar to 5 mu m, respectively. Two types of cells were made; YSZ layer between anode and GDC (GDC/YSZ) and YSZ layer between cathode and GDC (YSZ/GDC). Two platinum reference electrodes were embedded within the GDC layer. Cells were tested at 650 degrees C with hydrogen as fuel and air as oxidant. Electric potentials between embedded reference electrodes and anode and between cathode and anode were measured at open circuit. short circuit and under load. The electric potential was nearly constant through GDC in the cathode/YSZ/GDC/anode cells. By contrast, it varied monotonically through GDC in the cathode/GDC/YSZ/anode cells. Estimates of oxygen chemical potential, mu(O2), variation through GDC were made. mu(O2) within the GDC layer in the cathode/GDC/YSZ/anode cell decreased as the current was increased. By contrast, mu(O2) within the GDC layer in the cathode/YSZ/GDC/anode cell increased as the cutrent was increased. The cathode/YSZ/GDC/anode cell exhibited maximum power density of similar to 0.52 W cm(-2) at 650 degrees C while the cathode/GDC/YSZ/anode cell exhibited maximum power density of similar to 0.14 W cm(-2) for the same total electrolyte thickness. (C) 2009 Elsevier B.V. All rights reserved.