Processes And Related Materials Of Energy-converting Ceramic Membrane Cells

The most pressing challenges facing humankind today are environment and energy issues that both have direct links with our over dependence on finite fossil resources. Although clean energy, such as nuclear energy and solar energy, could be utilized in centralized area, efficient and clean energy fuel carriers are inevitably needed as well due to the different and specific situations of energy demands in the world. Clean fuels produced from CO2/H2O are an attractive alternative vector to hydrogen to carry renewable energy from where it is available to its point of use. Ceramic Membrane Electrolysis Cells have the efficiency as high as 90%or even higher than 100%to produce hydrogen via high temperature steam electrolysis. In this thesis, proton conducting and oxygen-ion conducting ceramic membrane electrolysis cells are utilized to generate clean fuels with CO2/H2O as precursors; the related materials and thin membrane fabrication are also investigated.The first chapter briefly introduces hydrogen and synthetic fuels playing as a bridge for clean energy utilizations and their production technology. The principles of ceramic membrane electrolysis cell and fuel cell are presented in detail; and the research progress of electrolyte and electrode materials is summarized in brief as well.There are several reports about high temperature electrolysis of H2O、CO2 and CO2/H2O, however, the ceramic membrane electrolysis cells are using Ni-based cathode which is not stable for cycling and needs significant content of hydrogen to maintain its metal state. In chapter 2,2-mm-thick YSZ disk supported ceramic membrane electrolysis cell (cathode)La0.8Sr0.2TiO3+δ/YSZ/La0.5Sr0.5MnO3-δ(anode) was utilized for H2O, CO2 and CO2/H2O (2:1) electrolysis to produce H2, CO and CO/H2, respectively. The efficiencies for three electrolysis together reach more than 80%with applied potential of 2 V. In addition, methane at the rate of 0.0047ml·min-1·cm-2 is directly generated for the first time when placing porous Fe catalyst onto cathode surface to combine Fishcher-Tropsch synthesis and co-electrolysis in one process based on a 0.7-mm-thick YSZ dick supported (cathode) La0.8Sr0.2TiO3+δ/YSZ/La0.5Sr0.5MnO3-δ(anode) at 650℃with applied potential of 2 V; H2/CO is produced as byproducts as well and the Faraday efficiency reaches 80%for the whole process. The electrode polarization resistance starts from 8Ω·cm2 at OCV condition, increases to 15Ω·cm2 at 0.4 V and then decreases to 1Ω·cm2 with applied potential of 1.8 V according to in-situ AC impedance.In the former part of chapter 3, hydrogen production via steam electrolysis is investigated in a cathode supported proton conducting ceramic membrane electrolysis cell (anode) (La0.75Sr0.25)0.95Mn0.5Cr0.5O3-δ/BaCe0.5Zr0.3Y0.16Zn0.04O3-δ/Ni (cathode) with 60-μm-thick electrolyte. The current reaches 2 A·cm-2 with applied potential of 2 V and also exhibits stable short-term performance at 700℃. The increase of electrolysis potential greatly decreases the electrode polarization resistance (Rp) according to in-situ AC impedance. Rp decreases from 20Ω·cm2 at OCV to 0.1Ω·cm2 at 2 V. In the later part of this chapter, CO2 is electrochemically reduced in a anode supported (anode) Ni/BaCe0.5Zr0.3Y0.16Zn0.04O3-δ/Fe (cathode) proton conducting ceramic membrane electrolysis cell with 60-μm-thick electrolyte at 614℃. The input for anode and cathode are 3%H2O/H/2 and 100%CO2 at the rate of 5 ml·min-1·cm-2, respectively. Methane (1.2%,0.07 ml·min-1·cm-2) is then directly generated for the first time from electrochemical reduction of CO2 with applied current of 1.5 A·cm-2. CO (61%) and H2 (8%) are also produced and the Faraday efficiency for whole process is 35%.Advanced materials and fabrication technology are crucial for high performance ceramic membrane cells. In chapter, in order to modify the chemical stability of barium cerate that is easy to react with CO2/H2O, we develop a novel ceramic proton conductor BaCe0.4Zr0.3Sn0.1Y0.2O3-δ(BSY) by co-doping Zr and Sn into the B site of BaCeO3. Results reveal that BSY exhibits desirable chemical stability after treatment in 3%CO2/3%H2O/94%Ar at 700℃for 24 h while Zr-replaced BaCe0.4Zr0.4Y0.2O3-δobviously decomposes under the same conditions. The conductivity of BSY reaches 0.009 S·cm-1 in 97%H2/3%H2O at 700℃. A single fuel cell with 20-μm-thick BSY electrolyte and Nd0.7Sr0.3MnO3-δ/BSY cathode fabricated by in-situ sintering and spray coating method exhibits maximum output of 320 mW·cm-2 and electrode polarization resistance of 0.33 ’Ω·cm2 at 700℃with 97%/3%H2O as fuel. In addition, we also fabricated reported BaCe0.7Zr0.1Y0.2O3-δ(BCZY) electrolyte membrane and the membrane formation mechanism is confirmed to be a combination of reaction sintering, liquid sintering and driven sintering. Single cell with 10-μm-thick BCZY electrolyte exhibits maximum output of 450 mW·cm-2 at 700℃.Advanced ceramic fuel cell cathode is mainly relying on Pervoskite cobalt-based materials, such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ; however, the long-term structure stability at intermediate temperature hinders widespread utilization. In chapter 5, we synthesize novel Ba0.5Sr0.5Al0.1Fe0.9O3-δ(BSAF) via solid state reaction method and characterize its structure stability and electrochemical performance. BSAF is cubic structure with space group of Fm3m and cell parameter of 0.39669 nm. The conduction in air is a combination of electron conduction (n type) and hole conduction (p type); p type conduction mainly dominates below 450℃while the n type conduction is main process above 450℃. The electrode polarization resistances in symmetric cell based on YSZ electrolyte are 2 and 1Ω·cm2 at 664℃and 727℃, respectively. In addition, in order to improve structural and chemical stability of cobalt-based cathode, we synthesized and characterized BaCo0.7Ta0.1Fe0.2O3-δ(BCTF). BCTF is cubic structure and keeps stable after treatment in 3%CO2/3%H2O/94%Ar at 700℃for 10 h; while BaCo0.8Fe0.2O3 is hexagonal structure and obviously decomposes under the same conditions. The conductivity of BCTF reaches 11 S·cm-1 at 700℃and the electrode polarization resistance is 0.1Ω·cm2 at 700℃in a symmetric cell based on a proton conducting BaCe0.7Zr0.1Y0.2O3-δelectrolyte.