Synthesis and Characterization of Layered Double Hydroxide Nanoparticles As Anion Conductors Toward Anion Exchange Membrane Water Electrolysis
Authors: Oosato Hirofumi , Mitsushima Shigenori , Kuroda Yoshiyuki 
Published: December 2024

Anion exchange membrane water electrolysis (AEMWE) is a promising technology because of the cost-effectiveness of catalysts and potential high energy efficiency.1 However, lifetime of electrocatalysts is limited by the degradation of anion exchange membrane and ionomer.2 To use layered double hydroxides (LDHs) as highly durable inorganic anion conductors for AEMWE, control of particle size and shape is important to design the structures of membranes and electrodes. LDH nanosheets are potentially useful for this purpose because they have large surface area to contact with catalysts and long ionic conduction path. Most LDH nanosheets are prepared by exfoliation of stacked LDHs in an aqueous solution. Because of the strong affinity of LDHs with carbonate, complete decarbonation is required for the process. Since decarbonation requires time and effort, simpler synthesis method is desirable for applying LDHs to AEM as ionomers. In this study, we demonstrate a novel method to synthesize LDH nanosheets, modifying their surface with tris(hydroxymethyl)aminomethane (Tris-NH2). Because the modification of LDH nanosheets suppresses their stacking, LDH nanosheets are obtained as a dispersion without the exfoliation process and decarbonation. The ionic conductivity of the samples was also measured, using an electrolyzer cell under the supply of liquid water.
MgAl LDH nanoparticles and nanosheets were synthesized by the reaction of aqueous solutions of metal salts and Tris-NH2 without decarbonation according to the literature for the synthesis of LDH nanoparticles.3 The nanoparticles and nanosheets with different particle sizes were synthesized by changing the concentration of Tris-NH2 and Mg/Al ratio in solution. Some of the obtained LDH nanoparticles and nanosheets were pelletized by uniaxial press at 450 MPa for 30 min. The ionic conductivities of the pellets were measured by electrochemical impedance spectroscopy (EIS) using a small electrolyzer cell with a precise pressure-control system for electrodes, using carbon papers as electrodes.
LDH nanoparticles were obtained with various Tris-NH2 concentrations and the constant Mg/Al ratio of 2.3. The size of the LDH nanoparticles decreased from 28 nm to 10 nm when the concentration of Tris-NH2 increased from 0.5 to 2 M. When the concentration of Tris-NH2 was constant at 4 M, the morphology of LDH materials changed from nanoparticles to nanosheets along with the increase in the Mg/Al ratio from 2.1 to 6.0. Fig. 1 is an image of nanosheets which were synthesized, using the concentration of Tris-NH2 of 4.0 M and the Mg/Al ratio of 6.0. It has been demonstrated in the literature that the amount of immobilized Tris-NH2 on Mg(OH)2 is higher than those on LDHs probably because the interactions between Tris-NH2 and Mg2+ are higher than those between Tris-NH2 and Al3+. Therefore, the LDH nanosheets were stabilized probably because a larger number of Tris-NH2 was immobilized on the basal plane of LDH sheets when the Mg/Al ratio increased. Thus, this method is useful for the one-pot synthesis of LDH nanosheets without exfoliation process under the exclusion of CO2.
Fig. 2 shows the Nyquist diagrams of three LDH nanoparticles 3, 10, and 28 nm in size, and a nanosheet several hundreds of nanometers or more in size. Each sample was named, using the morphology (nanoparticles: np, and nanosheets: ns), particle size (nm), and ion exchange capacity (meq cm–1) like np28nm_4.00 and ns_3.71. The Nyquist diagrams are standardized by the cross-sectional area (S) and thickness (t) of each pellet. Two partial semicircles were observed in the Nyquist diagrams, which was analyzed by the sequentially connected Randles-type equivalent circuit (Fig. 2, inset). The ionic resistance was characterized by R1. The ionic conductivities of np3nm_4.29, np10nm_4.18, and np28nm_4.00 were estimated to be 0.91, 0.29, and 1.38 mS cm–1, respectively. That of ns_3.71 was 0.48 mS cm–1. These values are consistent with those of LDHs in the literature.4 The newly synthesized LDH nanosheets are found to be usable as anion exchanger like conventional LDH nanoparticles.
In conclusion, LDH nanosheets were successfully synthesized without complex exfoliation and decarbonation processes, using Tris-NH2 as a surface stabilizer. The ionic conductivity of the samples was measured under the supply of liquid water like water electrolysis and the ionic conductivity of the LDH nanosheet was comparable to those of conventional LDH nanoparticles. This work was supported by the JSPS KAKENHI (grant number 24K01580).

References

S. Kumar and H. Lim, Energy Rep., 8, 13793 (2022).

A. Krivina et al., Adv. Mater., 34, 2203033 (2022).

Kuroda et al., Chem. Mater., 25, 2291 (2013).

S. Kim et al., Solid State Ionics, 181, 883 (2010).

Figure 1