A Superlattice of Alternately Stacked Ni–Fe Hydroxide Nanosheets and Graphene for Efficient Splitting of Water

Abstract

Cost-effective electrocatalysts based on nonprecious metals for efficient water splitting are crucial for various technological applications represented by fuel cell. Here, 3d transition metal layered double hydroxides (LDHs) with varied contents of Ni and Fe were successfully synthesized through a homogeneous precipitation. The exfoliated Ni-Fe LDH nanosheets were heteroassembled with graphene oxide (GO) as well as reduced graphene oxide (rGO) into superlattice-like hybrids, in which two kinds of oppositely charged nanosheets are stacked face-to-face in alternating sequence. Heterostructured composites of Ni2/3Fe1/3 LDH nanosheets and GO (Ni2/3Fe1/3-GO) exhibited an excellent oxygen evolution reaction (OER) efficiency with a small overpotential of about 0.23 V and Tafel slope of 42 mV/decade. The activity was further improved via the combination of Ni2/3Fe1/3 LDH nanosheets with more conductive rGO (Ni2/3Fe1/3-rGO) to achieve an overpotential as low as 0.21 V and Tafel plot of 40 mV/decade. The catalytic activity was enhanced with an increased Fe content in the bimetallic Ni-Fe system. Moreover, the composite catalysts were found to be effective for hydrogen evolution reaction. An electrolyzer cell powered by a single AA battery of 1.5 V was demonstrated by using the bifunctional catalysts.

Role of the synthesis route on the properties of hybrid LDH-graphene as basic catalysts

“…These self-assembly prepared hybrids also present additional weak harmonics, as well as (100), (110) and (113) peaks showing that the structure of the LDH nanosheets is maintained as recently reported in a HT/rGO hybrid. 16,17 . Besides, the small harmonics found in these samples at lower 2θ angles (marked by arrows in Figure 1A) reinforce the hypothesis of formation of a sandwichtype superstructure.…”

Highly selective charge-guided ion transport through a hybrid membrane consisting of anionic graphene oxide and cationic hydroxide nanosheet superlattice units

“…If the crystallographic thicknesses of GO (0.78 nm, shown in Figure 2d) and the LDH host layer (0.48 nm) are taken into account, a basal spacing of 1.26 nm is expected for a superlattice structure containing intercalated water, which is close to the value obtained here. The slightly reduced basal spacing can be attributed to the interlayer contraction originating from electrostatic attractions between the oppositely charged NS 46,47 and the loss of intercalated water in the interlayer galleries. The latter can be demonstrated by the relatively hydrophobic surface of the GO/LDH-NS hybrid membrane (Co-Al) (contact angle:~64°) compared with that of the GO membrane (contact angle:~44°).…”

“…[39][40][41][42][43][44][45] Because of the in-plane positive charges of LDH-NS, superlattice composites can be expected via alternative face-to-face assembly of cationic LDH-NS and anionic GO-NS on a molecular scale. 46,47 With these superlattice units, a macroscopic membrane may be constructed, in which the cationic LDH-NS are intercalated uniformly into the GO galleries, acting as both strengthening phases for electrostatic interactions and interlayer phases for modifying the physicochemical properties of the nanochannels. This unique structural feature may give rise to significant separations of small ions solely dependent on their charges.…”

Liquid Exfoliated Co(OH)2 Nanosheets as Low‐Cost, Yet High‐Performance, Catalysts for the Oxygen Evolution Reaction

“…[28,30,31] However, in the past, LDH nanosheets have been produced by relatively complex methods such as hydrothermal synthesis coupled with exfoliation by ion exchange. [6,7,24,32] Here we take a simpler approach, using LPE as a top-down method to produce Co(OH)2 nanosheets directly from the parent layered crystal. Liquid phase exfoliation is a simple, robust technique which can produce large volumes (>100s of litres) [25] of nanosheet dispersions in ambient conditions.…”