Effect of Ni in the Anode on the Performance of Pulse-Jet Rechargeable Direct Carbon Fuel Cells

NTRODUCTION

 Solid Oxide Fuel Cells (SOFCs) operate at high temperature, and thus various fuels can be used due to internal reforming reactions of the fuel. Our research group previously developed Rechargeable Direct Carbon Fuel Cells (RDCFCs) in which the solid carbon deposited by thermal decomposition of hydrocarbon is used as fuel [1, 2].

   We also developed Pulse Jet-Rechargeable Direct Carbon Fuel Cells (PJ-RDCFCs) as a new type of RDCFC [3, 4]. In a PJ-RDCFC, liquid hydrocarbon is supplied, thus enabling as fuel the use of hydrogen, lower hydrocarbons, and the solid carbon by thermal decomposition of the liquid hydrocarbon. The operating conditions of a PJ-RDCFC are broader because such conditions can be controlled by adjusting the fuel-supply interval time (T_int), the amount in a single pulse (PJ_vol), and the current density (j). The type of chemical species that react in a PJ-RDCFC can be selected by controlling these parameters [3]. Sulfur compounds contained in liquid fuel cause poisoning of the catalyst and increase the amount of deposited carbon, both of which cause catalyst degradation. Anode degradation can possibly be controlled, however, by adjusting the parameters in PJ-RDCFCs so that carbon-removal reactions occur selectivity [4].

   Traditionally, Ni/GDC cermet anodes are used in PJ-RDCFCs, but the amount of Ni should be kept at a minimum because of the formation of NiC_x. Only a slight difference existed between the power density (P) of RDCFCs with Ni and those without Ni, whereas the P of SOFCs with Ni/GDC was much smaller than those with GDC metal-free anodes [2]. In this research, to develop a metal-free anode that is tolerant against anode degradation, power generation characteristics of a PJ-RDCFC using a GDC metal-free anode and that using a Ni/GDC cermet anode were compared and the role of Ni in the anode was investigated.

EXPERIMENTAL

The electrolyte was a 8YSZ disk (0.25-mm thickness), the cathode was a (La_0.8Sr_0.2MnO₃)/ScSZ composite, and the anodes were a Ni/GDC (Gd_0.10Ce_0.90O_2-δ) composite and a GDC (Gd_0.20Ce_0.80O_2-δ:Gd_0.10Ge_0.90O_2-δ = 9:1) composite. Two different thicknesses of GDC were evaluated, 9- and 52-μm.

The operating temperature was 900 °C and oxygen was introduced to the cathode at 60 sccm. Hydrogen was introduced to the anode at 200 sccm during the power generation of the SOFCs. In the power generation of a PJ-RDCFC, after 3 jet-pulses of isooctane as fuel followed by a 1-min wait, power generation at constant j was initiated. PJ_vol was set to 1.2 μl. Charging was again carried out by pulse jetting at fixed T_int (60 sec). After 15 power generation/charging cycles, pulse jetting was stopped.

RESULT AND DISCUSSION

  Figure 1 shows the current density-terminal voltage (I-V) characteristics of the SOFCs fueled by hydrogen using (1) Ni/GDC (42-μm), (2) GDC (52-μm thickness) and (3) GDC (9-μm thickness). Their respective maximum P was (1) 225 mW/cm², (2) 47.9 mW/cm², (3) 132 mW/cm²_, and ohmic resistance was (1) 0.324 Ω/cm², (2) 3.47 Ω/cm², (3) 1.06 Ω/cm². Lower electric conductivity in GDC leads to high ohmic resistance, and thus the maximum P using GDC anodes here was lower than using Ni/GDC. Ohmic resistance decreased, however, with the thinner GDC anode, resulting in a higher P.

 Figure 2 shows the power generation characteristics of PJ-RDCFCs using (1) Ni/GDC at j = 100 mA/cm² and (2) GDC (9-μm thickness) at j = 15 mA/cm². The average P were (1) 77.8 mW/cm² and (2) 11.0 mW/cm². The same V behavior was exhibited during each power generation of a single pulse (i.e., cyclic power generation), showing that steady power generation occurred using a GDC metal-free anode. With the Ni/GDC anode, V increased during a cyclic power generation, whereas with GDC, it increased just after pulse jetting and then decreased. In PJ-RDCFCs, V is affected by changes in the chemical species that contribute to the electrochemical and thermochemical reactions following increasing fuel utilization. Because Ni is thought to be mainly the catalyst of thermochemical reactions, the difference in power generation behavior with these two anodes apparently is due to the difference in such contribution by Ni to the thermochemical reactions.

  In conclusion, power generation of PJ-RDCFCs using a GDC metal-free anode was successful, and suggested that the difference in power generation characteristics is due to the presence of Ni, which is a catalyst of thermochemical reactions.

REFERENCES

[1]M. Ihara et al., Solid State Ionics, 175, 51 (2004)

[2]Y. Tagawa et al., ECS Trans., 25 (2), 1133 (2009)

[3] S. Sugiyama et al., ECS Trans., 58 (45), 21 (2014)

[4] S. Hattori et al., ECS Trans., 64 (2), 211 (2014)