The relationship between the electrochemical performance of an anode in a solid oxide fuel cell and the distribution of yttrium-doped barium zirconate (BZY) in the electrochemically active zone of nickel (Ni)/yttria-stabilized zirconia (YSZ) anodes was investigated to clarify the promoting effect of BZY on electrochemical reaction. After BZY was injected into Ni/YSZ anodes using an ink-jet technique for control of the distribution of BZY, the anode performance improved in both humidified H 2 and dry CH 4 fuels. The performance of the BZY-infiltrated Ni/YSZ anodes depended on the distribution of BZY; an increasing gradient of BZY from the electrolyte side to the anode surface side was particularly effective in improving the performance. These results suggest that the optimum amount of infiltrated BZY to enhance the electrochemical reaction depended on the distribution of the oxygen chemical potential in the Ni/YSZ anodes. According to the competitive adsorption equilibrium of reactants and the Langmuir-type reaction on the surface of Ni at the triple-phase boundary, the oxygen coverage was a critical factor in the electrochemical reaction and depended on the oxygen chemical potential. In conclusion, the enhanced electrochemical reaction in the BZY-infiltrated Ni/YSZ anodes was therefore due to the increased oxygen coverage.
The dependence of electrode performance on the distribution of proton conductor yttrium-doped barium zirconate (BZY) in nickel (Ni)/yttria-stabilized zirconia (YSZ) anodes was investigated to clarify the role of BZY in improving the anode performance in dry methane fuel. In general, electrochemical, decomposition, carbon removal, and reforming reactions occur as anode reactions, and the electrochemical reactions occur only inside the electrochemically active zone of anodes, although the other reactions can also occur outside the electrochemically active zone. To determine the reaction enhanced by BZY addition, BZY was distributed either inside or outside electrochemically active zone of Ni/YSZ anodes by using double-layer anodes consisting of Ni/YSZ and Ni/YSZ/BZY layers. In the Ni/YSZ/BZY layer, BZY was evenly and finely dispersed using a composite powder prepared by spray pyrolysis. The addition of BZY inside the electrochemically active zone improved the anode performance, whereas the addition of BZY outside the electrochemically active zone did not, suggesting that the main role of BZY was to enhance the electrochemical reactions.
Rechargeable Direct Carbon Fuel Cells (RDCFC) with Ni-cermet anodes and oxide anodes were developed. An RDCFC with a Ni/GDC (gadolinium-doped ceria) anode and 0.3-mm-thick ScSZ (scandium-stabilized zirconia) electrolyte with propane as the charging gas for 5 min at 1173 K exhibited a maximum power density of 258 mW/cm 2 , which is a 5 times improvement over a previously reported similar RDCFC except with YSZ electrolyte. To investigate the reaction mechanism in RDCFCs and the role of Ni, the power generation of RDCFCs and conventional SOFCs with Ni/GDC, Ni/ScSZ, GDC, and ScSZ/GDC anodes was evaluated. The results showed that (a) GDC/ScSZ had greater amount of charged carbon than GDC anode, (b) RDCFCs with Ni/ScSZ anode showed extremely poor power generation performance, whereas the SOFC with Ni/ScSZ showed good performance when fueled either by hydrogen or dry methane, (c) carbon deposition by propane did not strongly depend on the presence of Ni under propane charging studied here, and (d) RDCFCs with metal-free anodes, namely, GDC and GDC/ScSZ anodes, exhibited higher power generation performance than did similar SOFCs fueled by dry methane.
Rechargeable Direct Carbon Fuel Cells (RDCFC) with Ni/GDC-cermet anodes were developed. RDCFCs with a Ni/GDC (gadolinium-doped ceria) anode and 0.3-mm-thick ScSZ (scandia-stabilized zirconia) electrolyte charged with propane for 5 min at 600° exhibited a stable, maximum power density of 200 mW/cm2 for more than 1 h at operating temperature of 900°. The effect of charging temperature on the properties of charged carbon and on power generation was studied here. When the charging temperature was decreased from 900° to 750°, power generation time increased by more than a factor of 10, from 22 to 280 min. Based on the TG-DTA results, carbon charged at 600° could easily be oxidized, and CO supply by the Boudouard reaction was improved. This led to an improved power density and fuel utilization.
Power generation tests of pulse jet (PJ) rechargeable direct carbon fuel cells (PJ-RDCFCs), which supply small amounts of isooctane to the anode by pulse jetting, were conducted at different PJ supply frequency and for different PJ supply amount. PJ-RDCFCs could control power density by controlling the supply frequency. At 1 pulse/sec, power generation characteristics of PJ-RDCFCs approached flow-type. At lower supply frequency, power generation characteristics approached batch-type and fuel utilization becomes higher. Fuel utilization also became higher with decreasing PJ supply amount. In the reaction mechanism of a PJ-RDCFC, the contributing species for power generation might be hydrogen, carbon monoxide and hydrocarbons when the PJ supply frequency is high (≈1 pulse/sec). With decreasing supply frequency, carbon which has lower reactivity became to contribute to the power generation. In conclusion, the electrochemical reactions in the anode side in a PJ-RDCFC can be controlled by adjusting the pulse jet supply frequency.
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