Development of Alkaline Fuel Cells Using Hydroxide-Ion Conductive Layered Double Hydroxides

Alkaline fuel cells (AFCs) with solid electrolytes such as anion exchange membrane (AEM) have received attention in recent years. This is mainly due to faster kinetics for oxygen reduction reaction (ORR), and the possible use of non-platinum catalyst such as nickel and silver in the alkaline fuel cells. In the traditional AFCs using KOH aqueous solution as the electrolyte, the electrolyte is very sensitive to the presence of carbon dioxide. However, in the AFCs with solid electrolyte, a degradation with carbon dioxide can be avoided.

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  [M^II_1-x M^III_x(OH)₂][(A^n-)_x/n·mH₂O], where M^II is a divalent cation such as Ni²⁺, Mg²⁺, Zn²⁺, etc., and M^III is a trivalent cation such as Al³⁺, Fe³⁺, Cr³⁺, etc., and A^n- is an anion such as CO₃^2-, 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 CO₃^2- 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 CO₃^2-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 [4].  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.

[1] K. Tadanaga, Y. Furukawa, A. Hayashi, M. Tatsumisago: Adv. Mater. 22(2010) 4401.

[2] Y. Furukawa, K. Tadanaga, A. Hayashi, M. Tatsumisago, Solid State Ionics 192(2011) 185.

[3] D. Kubo, K. Tadanaga, A. Hayashi, M. Tatsumisago: J. Electroanal. Chem. 671(2012) 102.

[4] D. Kubo, K. Tadanaga, A. Hayashi, M. Tatsumisago: J. Power Sources 222(2013) 493.