Electrochemical Characterization of a Solid Oxide Membrane Electrolyzer for Production of High-Purity Hydrogen
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INTRODUCTION
THE annual energy consumption is approximately 450 quadrillion BTU and is expected to double in the next 40 years.[1] Most of this energy (~85 pct) is obtained from nonrenewable carbon-based sources with no means for CO2 sequestration. This process has contributed to global warming because of the increase in content of the atmospheric CO2. Hydrogen-based energy economy from carbon-neutral energy sources has been envisioned as one of the potential solutions to this crisis.[2] Currently, almost all the hydrogen is produced by the reformation of methane with steam or coal gasification reactions with steam, and the use is limited to ammonia production and petroleum refining.[3] A cleaner, reliable, cheaper hydrogen production route that is independent of fossil fuel is necessary for hydrogen to be used extensively as an energy carrier. Another process to produce pure hydrogen is electrolysis of water at a high temperature using a solid oxide steam electrolyzer (SOSE) based on an oxygenconducting stabilized electrolyte.[4] In a SOSE, a mixture SOOBHANKAR PATI, Graduate Student, KYUNG JOONG YOON, Postdoctoral Research Associate, SRIKANTH GOPALAN, Associate Professor, and UDAY B. PAL, Professor, are with the Division of Materials Science and Engineering, Department of Mechanical Engineering, Boston University, 15 Saint Mary’s Street, Brookline, MA 02446. Contact e-mail: [email protected]. Manuscript submitted January 28, 2009. Article published online September 1, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
of steam and hydrogen is fed to the cathode and air is circulated over the anode. At the cathode–electrolyte interface, steam is dissociated, which employs an external electrical power source to form hydrogen and oxygen ions. The oxygen ions migrate across the electrolyte and are oxidized to form gaseous oxygen at the anode–electrolyte interface. The overall reaction of the SOSE is as follows: 2H2 O ðgÞ ¼ 2H2 ðgÞ þ O2 ðgÞ
½1
The open-circuit/Nernst potential at 1000 C of the SOSE with a gas composition of 90 pct H2O-H2 at the cathode and air at the anode is 0.75 V. The applied potential across the cathode and the anode must be greater than the open-circuit potential to drive the reaction (Eq. [1]) forward and produce hydrogen. In a conventional SOSE, 60 to 70 pct of the electric power is used to overcome this energy barrier to drive the reaction forward.[4] The open-circuit potential (energy barrier) and, thus, the electrical energy consumption can be reduced by reducing the oxygen chemical potential at the anode using a reductant. In previous studies, researchers have proposed using CO and natural gas to decrease the anodic oxygen chemical potential in a SOSE.[5–7] However, the conventional SOSE anode cannot handle solid waste as a reductant feed, because the solid anode in the SOSE does not provide sites to perform charge transfer simultaneously while oxidizing the reductant feed. In this article, a solid VOLUME 40B, DECEMBER 2009—1041
Fig. 2—Configuration of the SOM electrolyzer using reductant (R) in
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