Poisoning Effect of Sulfur Compounds in Pulse Jet-Rechargeable Direct Carbon Fuel Cells

INTRODUCTION

     Rechargeable Direct Carbon Fuel Cells (RDCFCs) use the solid carbon deposited on the anode by thermal decomposition of hydrocarbons as fuel [1-4]. One application of RDCFCs is Pulse Jet-Rechargeable Direct Carbon Fuel Cells (PJ-RDCFC) [5]. In a PJ-RDCFC, small amounts of a liquid hydrocarbon are supplied via pulse jet, thus allowing not only the solid carbon to be used as fuel but also hydrogen, methane and lower hydrocarbons by thermal decomposition of the liquid hydrocarbon. Operating conditions of PJ-RDCFCs can be controlled by adjusting the interval time of the pulse jet (T_int), the amount in a single pulse (PJ_vol) and the current density (j). Our group previously suggested that the chemical species contributing to an electrochemical reaction in PJ-RDCFCs can be controlled by adjusting T_int, PJ_vol and j[5].

     Kerosene and gasoline are typical liquid hydrocarbon fuels that contain sulfur-based impurities, such as benzothiophene and its derivatives. Sulfur compounds are generally known to degrade the fuel anode in SOFCs because they can poison the catalyst of the anode. Some studies reported that the power density in SOFCs decreases dramatically when the supplied hydrogen contains 50 ppm hydrogen sulfide [6,7]. However, power generation characteristics of PJ-RDCFCs have not been evaluated when the supplied liquid hydrocarbon contains sulfur.

     In this study, the poisoning effect of sulfur compounds on the power generation characteristics of PJ-RDCFCs was investigated by measuring and then comparing these characteristics when the fuel was isooctane (the main component of gasoline) with and without benzothiophene.

EXPERIMENTAL

     In the PJ-RDCFC used here, the electrolyte was an YSZ disk (8 mol% Y₂O₃ –stabilized 1 mol% ZrO₂, 0.25-mm thickness, 20-mm diameter), the anode was a Ni/Gd_0.10Ce_0.90O_2-δ composite, and the cathode was a La_0.8Sr_0.2MnO₃/ScSZ composite.

     The operating temperature was 900ºC and oxygen was introduced to the cathode at STP 60 ccm. After 3 jet-pulses of fuel followed by a 1-min wait, power generation at constant j was initiated. Charging was again carried out by pulse jetting at fixed T_int. After two or more and power generation/charging cycles, pulse jetting was stopped, and power generation was considered done when terminal voltage Vreached 0 V.

     First, power generation tests using isooctane without benzothiophene were performed at T_int = 30 sec, PJ_vol = 1.2 μl and j = 100 and 200 mA/cm². Then, power generation tests using isooctane with 100 ppm sulfur adjusted by benzothiophene addition was performed at T_int = 30 sec, PJ_vol = 1.3μl and j = 100 and 200 mA/cm².

RESULTS AND DISCUSSION

     Figure 1 shows V as a function of time at j = 100 mA/cm² and at T_int = 30 sec. For 15 minutes from the start of power generation, 30 pulses of fuel were supplied. After 15 minutes, residual fuel might have been either reformed or decomposed. When isooctane with sulfur was supplied, the average power density was 66.9 mW/cm², a 13% decrease compared with that when only isooctane was supplied (76.7 mW/cm²⁾. In addition, the power generation time was shorter when the supplied fuel was isooctane with sulfur 33.4 minutes than isooctane only 37.6 minutes.

     Figure 2 shows V as a function of time at j = 200 mA/cm² and at T_int = 30 sec for a PJ-RDCFC using supplied isooctane with and without 100 ppm sulfur. For 20 minutes from the start of power generation, 40 pulses of fuel were supplied. After 20 minutes, residual fuel might have been either reformed or decomposed. When isooctane with sulfur was supplied, the average power density was 110 mW/cm², which is a 21% decrease compared with that when only isooctane was supplied (139 mW/cm²).

     In conclusion, the power density during power generation of PJ-RDCFCs under the normal operational conditions is lower when sulfur compounds are in the supplied liquid hydrocarbon. However, because PJ-RDCFCs can control electrochemical reactions contributing to the power generation, we can expect to find some operational conditions to prevent the degradation due to sulfur compounds.

REFERENCES

[1]M. Ihara et al., Solid State Ionics, 175, 51-54 (2004)

[2]S. Hasegawa et al., J. Electrochem. Soc., 155, B58-B63 (2007)

[3]Y. Tagawa et al., ECS Trans., 25(2) 1133-1142 (2009)

[4]F. Ohba et al., ECS Trans., 33(40), 163 (2011)

[5]A. Yabuki et al., ECS Trans., 41(12) 57-67 (2012)

[6] S. Zha, et al., J. Electrochem. Soc., 154 (2) B201-B206 (2007)

[7] K. Sasaki, et al., J. Electrochem. Soc., 153 (11) A2023-A2029 (2006)