Impact of cation redox chemistry on continuous hydrothermal synthesis of 2D-Ni(Co/Fe) hydroxides

Abstract

Continuous hydrothermal flow synthesis (CHFS) is a facile, upscalable and costefficient synthetic method enabling the nanostructuring of advanced functional materials in steady conditions, i.e. not in batch synthesis. In this paper, we use CHFS to crystallize NiCo-and NiFe-Hydroxides in water solution with 2D nanofeatures. By tuning the synthetic parameters, we disclose the key role of the cation redox chemistry in the transition between two competitive phases: from 2D-nanoplatelets of brucite to layered double hydroxides (LDH). For controlling the precipitation of different Ni,Fe,Co-Hydroxide phases, we propose the combined use of an oxidizing (H 2 O 2 ) and a complexing agent (NH 3 ). At temperatures as low as 80 °C, the presence of H 2 O 2 and a low concentration of NH 3 favour the Ni 2+ /Co 3+ over Ni 2+ /Co 2+ oxidation states, shifting the product structure from brucite phase (temperatures > 80 °C) to LDH. Conversely, for the NiFe-Hydroxides the transition from LDH (temperatures ≤ 80 °C) to brucite phase (temperatures > 80 °C) is controlled by the reaction temperature only. Due to the high stability ofFe 3+ , the synthesis of NiFe products by CHFS does not require oxidizing and complexing agents, resulting in a robust process for large-scale production.

Assembling Ni–Fe Layered Double Hydroxide 2D Thin Films for Oxygen Evolution Electrodes

“…The XRD pattern in Fig. 1c confirmed the hydrotalcite-like structure exhibited by LDH materials with typical (003) and (006) basal peaks 42,45 . The combination of these results led to the univocal determination of the phase as Ni-Fe LDH (Fig.…”

“…The XRD pattern in Figure 1C confirmed the hydrotalcite-like structure exhibited by LDH materials with typical (003) and (006) basal peaks. 42,45 The combination of these results led to the univocal determination of the phase as Ni−Fe LDH (Figure 1D). The different broadening of the diffractogram peaks provided additional structural information.…”

“…The products were eventually depressurized and cooled down at room temperature for collection. The obtained products were purified by centrifuging and re-dispersing the particles in clean DI water several times 42 .…”

“…The obtained products were purified by centrifuging and redispersing the particles in clean DI water several times. 42 The obtained dispersion, referred to in the next sections as "CHFS dispersion", had a pH of 9 and was used as the starting point for the formulation of the inkjet printable ink ("LDH ink") (vide inf ra).…”