Ceramics improve operating conditions of solid-oxide fuel cells

  • PDF / 256,216 Bytes
  • 2 Pages / 585 x 783 pts Page_size
  • 93 Downloads / 262 Views

DOWNLOAD

REPORT




Energy Sector Analysis

The technology is being used to deliver electricity with high efficiency at residences and office buildings worldwide.

Ceramics improve operating conditions of solid-oxide fuel cells By Melissae Fellet Feature Editor Wolfgang Rossner

S

olid-oxide fuel cells (SOFCs) convert chemical energy into electricity, at higher efficiencies and with less emissions than conventional generators. The chemical energy comes from the reaction of fuel, which can be natural gas, gasoline, diesel, biofuel or hydrogen, with oxygen ions produced from air. SOFC generators are used to power homes, office buildings, and shopping centers worldwide. The units also produce heat, and when that heat is captured and reused, these fuel cells can be up to 85% efficient. In contrast, plant scale efficiency of electricity produced from coal and natural gas in the United States is about 30%. SOFCs are most efficient when they operate at high temperatures, originally around 800 to 1000°C. This is because the electrochemical processes inside the cell are thermally driven. The higher the temperature, the faster the reactions and transport rates, and the more current produced. But high operating temperatures also enhance intrinsic mechanical and structural degradation of the materials inside the cell, thus decreasing the SOFC lifetime. According to the September 2014 issue of the MRS Bulletin, the challenge of current SOFC research is to create a cell that can operate at lower temperatures (below 650°C), without sacrificing performance or reliability. Lowering the operating temperature of a SOFC also means some ceramic and expensive high chromium steel components could be replaced with ferritic stainless steel that would oxidize at higher temperatures. This replacement would reduce the cost of a cell, because metal is cheaper and easier to manufacture than ceramic. A SOFC contains a solid electrolyte sandwiched between two electrodes, the air electrode and the fuel electrode. At the air electrode, or the cathode, oxygen gas is reduced to oxygen ions, O2–. Those ions migrate through the ceramic electrolyte to the fuel electrode, the anode. At this electrode, the oxygen ions react with hydrogen and carbon monoxide, produced from reformed fuel. The oxidation reactions form steam and small amounts of carbon dioxide, about one-third less CO2 per kilowatt-hour than internal combustion engines. This step also generates electricity. Yttria-stabilized zirconia was used in the late 1930s as the electrolyte in the first SOFC, and it is still the most common electrolyte today. This material conducts oxygen ions very well at elevated temperatures, without transporting electrons that would short circuit the cell and decrease cell efficiency. It also has good stability toward the reducing environment of the anode on one side and the oxidizing environment of the cathode on the other. However, its conductivity drops as the temperature is

Wolfgang Rossner, Siemens, Germany Melissae Fellet, [email protected]

214

MRS BULLETIN



VOLUME

Data Loading...