Development of Alkaline Fuel Cells Using Hydroxide-Ion Conductive Layered Double Hydroxides
We have focused attention on layered double hydroxides (LDH) as an ion conducting material. LDHs are anionic clay and the general formula for LDHs is [MII1-x MIIIx(OH)2][(An-)x/n·mH2O], where MII is a divalent cation such as Ni2+, Mg2+, Zn2+, etc., and MIII is a trivalent cation such as Al3+, Fe3+, Cr3+, etc., and An- is an anion such as CO32-, Cl-, OH-, etc. We have recently reported that the ionic conductivity of LDHs were closely related to the species of intercalated anions, LDHs intercalated with CO32- showed high hydroxide ion conductivity of the order of 10-3 S cm-1 under 80% relative humidity [1-3]. We also reported that LDHs were hydroxide ion conductor, and can be applied to the solid electrolyte of alkaline direct ethanol fuel cell (DEFC) [1, 3].
In a fuel cell, the formation of good triple phase boundary (TPB) in the electrodes is very important. Although Nafion acts as proton conducting ionomer and drastically improves the cell performance in proton-conductive polymer electrolyte fuel cells, sufficient materials, ionomers, to effectively transport OH-ions, have not been found in AFC at present.
In this presentation, the ionic conductivity of M-Al LDHs (M=Ni, Mg, (Ni, Mn)) and M-Fe LDHs (M=Ni, (Ni, Mn)) intercalated with CO32-will be presented. Application of LDHs to the electrolyte or catalyst layer of the fuel cells will also be reported.
LDHs stuided in the present study are found to show high ionic conductivity of the order of 10-3 S cm-1 under the rerative humidity 80%. The results in EMF measurements for a water vapor concentration cell showed that these LDHs are a hydroxide ion conductor under the humidified condition.
H2-O2 fuel cells with Mg-Al and Ni-Al LDHs as an electrolyte and MnO2 as the cathode catalyst were confirmed to be operated. Alkaline-type DEFC using anion exchange membrane as an electrolyte and the electrodes with LDH as “ionomer” was also fabricated . The DEFCs using catalyst layers with Ni-Al, Mg-Al, Ni-Fe, and (Ni, Mn)-Fe LDHs showed higher cell performance than the DEFC using catalyst layer without LDH. These results indicate that hydroxide ion conducting LDHs work as ionomer, and construct more favorable TPB regions in the catalyst layer.
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