Direct Synthesis and Anion Exchange of Noncarbonate-Intercalated NiFe-Layered Double Hydroxides and the Influence on Electrocatalysis

Abstract

Metal-layered double hydroxides (LDHs), a family of versatile layered materials recently popular as earth-abundant oxygen evolution reaction (OER) catalysts, allow for both metal substitution and interlayer anion exchange without disrupting the two-dimensional (2D) crystal structure, although the ubiquity of carbonate anions is challenging to avoid. Here, we use hydrothermal synthesis in modified solvents with triethanolamine to directly synthesize bulk NiFe LDHs with carbonate, chloride, and sulfate interlayer anions without inert atmosphere protection. Structural characterizations by X-ray diffraction, infrared spectroscopy, and energy-dispersive X-ray spectroscopy confirm the anion composition. Electrochemical characterization shows that LDHs with these three anions display similar OER catalytic performance after surface area normalization, consistent with observed anion exchange to carbonate in alkaline electrolyte under ambient conditions. However, dodecyl sulfate-intercalated NiFe LDHs prepared by anion exchange show enhanced electrocatalytic performance unexplained by surface area and resist carbonate exchange during electrochemical cycling. The synthetic methods developed for carbonate, chloride, and sulfate LDHs also allow direct fabrication of three-dimensional high surface area electrodes, but the anion composition of these electrodes does not affect the OER performance after surface area normalization. These results provide new insights on how the interlayer anions in LDH materials influence OER catalytic activity and open a new approach for tuning these catalytic materials.