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We have developed high-performance double-layer (DL) hydrogen electrodes for reversible solid oxide cells. The DL hydrogen electrode consisted of mixed conductor, samaria-doped ceria (SDC), with highly dispersed Ni or Ni-Co nanocatalysts as the catalyst layer (CL) and, on top of it, a thin Ni−SDC cermet as the current collecting layer (CCL). The performance of the DL hydrogen electrode was appreciably improved by controlling the microstructure. The use of a thin, porous CCL increased the electronic conducting path to and from the CL, while maintaining sufficient gas-diffusion rates of H 2 and H 2 O, and enlarging the effective reaction zone at the CL. The optimum CCL thickness was found to be 5 μm. The IR-free overpotentials η at the optimized DL hydrogen electrode in humidified hydrogen (p[H 2 O] = 0.4 atm) and T cell = 800 • C were 0.20 and −0.20 V at j = 0.5 and −0.5 A cm −2 , respectively, indicating a highly reversible operation. The use of a full cell with the configuration of Ni 0.9 Co 0.1 /SDC DL hydrogen electrode|YSZ electrolyte|SDC interlayer|LSCF−SDC O 2 electrode led to very promising results for the SOEC operation in which an IR-free electrolytic cell potential of 1.21 V at j = −0.5 A cm −2 and 800 • C was achieved. Nowadays, hydrogen is attracting much attention as an energy carrier suitable for long-term and large-scale storage. Electrolysis of water is one of the effective technologies to produce pure hydrogen in a single step. Among the various types of electrolysis cells, solid oxide electrolysis cells (SOECs) operating at high temperature (800-1000• C) present the highest conversion efficiency due to favorable thermodynamic and kinetic conditions. 1-7 An SOEC can be operated in reverse mode as a solid oxide fuel cell (SOFC) that directly converts the chemical energy in hydrogen gas into electrical energy with a high energy conversion efficiency. Thus, a reversible solid oxide cell (R-SOC) is regarded as an efficient reciprocal energy converter between hydrogen and electricity. [8][9][10][11][12] Since the pioneering work in late 1960s to nowadays, the materials commonly used in SOFCs have been swiftly adopted in SOECs, i.e., yttria-stabilized zirconia (YSZ) electrolytes, Ni-YSZ cermet hydrogen electrodes, and perovskite-type oxygen electrodes based on La 1−x Sr x MnO 3−δ (LSM) or La 1−x Sr x Fe 1−y Co y O 3−δ (LSCF).1-11 However, it is very important to develop high-performance electrodes for R-SOC as clearly demonstrated by recent modeling or calculation.9,12 The essential factors for improving electrode performance are a high electrocatalytic activity and an extended effective reaction zone (ERZ).9,13 The ERZ is located around the physical triple phase boundary (gas/oxide ion conductor/electronic conductor). In the case of Ni-YSZ cermet hydrogen electrode, for example, the polarization performance (the extension of the ERZ) strongly depends on the electrode microstructure (size and distribution of Ni and YSZ, their connectivity, thickness and porosity), which is closely related to the fabri...

We have examined the durability of oxygen and hydrogen electrodes for reversible solid oxide cells (R-SOCs). The use of a dense, uniform samaria-doped ceria (SDC) interlayer was very important to obtain high performance and high durability of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) -SDC composite oxygen electrode. Double-layer hydrogen electrodes, consisting of SDC scaffold with highly dispersed Ni-Co nanoparticles as the catalyst layer (CL) and a thin current collecting layer (CCL) of either Ni-SDC or Ni-YSZ cermet, exhibited nearly identical initial performance at 800°C, while the use of Ni-YSZ CCL together with threedimensional network of Ni in the CL resulted in higher durability.

We have developed high performance electrodes for reversible SOEC/SOFCs. Double-layer hydrogen electrodes, consisting of a mixed conducting samaria-doped ceria (SDC) with highly dispersed Ni–Co catalysts as the catalyst layer (CL) and, on top of it, a thin Ni–SDC cermet as the current collecting layer (CCL), exhibited highly reversible performance at 800°C by controlling the microstructure. We have also succeeded in enhancing the reversible performances of LSCF–SDC composite oxygen electrodes accompanied by high durability.

Within the framework of developing reversible solid oxide fuel cells/solid oxide electrolysis cells (SOFCs/ SOECs), we have engaged in the design of high-performance electrodes. Facile production of hollow spherical particles of samaria-doped ceria (SDC) decorated with Ni-Co alloy catalysts (Ni 100ÀX Co X /SDC, X ¼ 0-50 atom%), nanometers in size, was adopted. A distinctive microstructure was observed in the hydrogen electrode fabricated with this composite powder. The cobalt addition played a significant role in evolution of the microstructure and effectively enlarged the electrochemical reaction zone. Under SOFC/SOEC reversible operation, the optimal performance characterized by a comparatively low ohmic resistance and high electrocatalytic activity was achieved with the Ni 80 Co 20 /SDC composition. At 900 C and an overpotential of 0.1 V, the current density reached remarkably high values of 0.18 and 0.37 A cm À2 in SOFC and SOEC operation, respectively.