Electrochemical Oxygen Separation Using Layered Double Hydroxides As Hydroxide Ion Conductor

Oxygen separation membranes using oxide ion conducting materials have been studied for oxygen separation from air.  However, the membrane can effectively separate oxygen from the air only at high temperatures higher than several hundred degrees, because the oxide ion conductor shows high ionic conductivity at only elevated temperatures.

On the other hand, we have been studied layered double hydroxide (LDH) as a hydroxide ion conducting material [1-3].  LDHs are consisting of positively charged metal hydroxide layer and interlayer anions for charge compensation of cationic layer. The general formula for LDHs is [M^II_1−x M^III_x(OH)₂][(Aⁿ⁻)_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 Aⁿ⁻ is an anion such as CO₃²⁻, Cl⁻, OH⁻, etc.  We have recently reported that LDHs intercalated with CO₃^2- showed high ionic conductivity of the order of 10^-3 S cm^-1under humidification, and the LDHs are solid hydroxide ion conductors [1-3]. 

Here we report the development of an electrochemical oxygen separation process using a hydroxide ion conductor [4].   The electrochemical oxygen separation cell consists of pelletized OH⁻ conductive solid electrolyte and electrodes.  At the air feed side, oxygen in air, H₂O and electrons produces OH^- ions at the electrode with the electrochemical oxygen reduction reaction.  At the same time, oxygen, H₂O and electrons are generated from OH⁻ ions at the other electrode with the electrochemical oxygen evolution reaction. Since H₂O will easily be separated with O₂, oxygen separation will be achieved.

In this study, Ni-Fe LDH intercalated with CO₃²⁻ (Ni-Fe CO₃²⁻ LDH) was used as a hydroxide ion conductive material.  The pellet of Ni-Fe CO₃²⁻ LDH as an electrolyte was sandwiched with electrodes using Pt/C as a catalyst and Ni-Fe CO₃²⁻ LDH as an ionomer. The oxygen concentration at the O₂ product side was determined using a gas chromatography system.  The electrochemical oxygen product flow was confirmed to be proportional to the external current density at 50^oC under 70% of relative humidity, indicating that the membrane functioned as electrochemical oxygen separator.

[1] K. Tadanaga, Y. Furukawa, A. Hayashi, M. Tatsumisago, Adv. Mater., 22 (2010) 4401-4404.

[2] Y. Furukawa, K. Tadanaga, A. Hayashi, M. Tatsumisago, Solid State Ionics, 192(2011) 185-187.

[3] D. Kubo, K. Tadanaga, A. Hayashi, M. Tatsumisago, J. Electroanal. Chem., 671 (2012) 102-105.

[4] Y. Arishige, D. Kubo, K. Tadanaga, A. Hayashi, M. Tatsumisago, Solid State Ionics, 262 (2014) 238-240.