Molecular-Scale Manipulation of Layer Sequence in Heteroassembled Nanosheet Films toward Oxygen Evolution Electrocatalysts

Abstract

Flocculation or restacking of different kinds of two-dimensional (2D) nanosheets into heterostructure nanocomposites is of interest for the development of high-performance electrode materials and catalysts. However, lacking a molecular-scale control on the layer sequence hinders enhancement of electrochemical activity. Herein, we conducted electrostatic layer-by-layer (LbL) assembly, employing oxide nanosheets (e.g., MnO2, RuO2.1, reduced graphene oxide (rGO)) and layered double hydroxide (LDH) nanosheets (e.g., NiFe-based LDH) to explore a series of mono- and bilayer films with various combinations of nanosheets and sequences toward oxygen evolution reaction (OER). The highest OER activity was attained in bilayer films of electrically conductive RuO2.1 nanosheets underlying catalytically active NiFe LDH nanosheets with mixed octahedral/tetrahedral coordination (NiFe LDHTd/Oh). At an overpotential of 300 mV, the RuO2.1/NiFe LDHTd/Oh film exhibited an electrochemical surface area (ECSA) normalized current density of 2.51 mA cm–2 ECSA and a mass activity of 3610 A g–1, which was, respectively, 2 and 5 times higher than that of flocculated RuO2.1/NiFe LDHTd/Oh aggregates with a random appearance of a surface layer. First-principles density functional theory calculations and COMSOL Multiphysics simulations further revealed that the improved catalytic performance was ascribed to a substantial electronic coupling effect in the heterostructure, in which electrons are transferred from exposed NiFe LDHTd/Oh nanosheets to underneath RuO2.1. The study provides insight into the rational control and manipulation of redox-active surface layers and conductive underlying layers in heteroassembled nanosheet films at molecular-scale precision for efficient electrocatalysis.

Monolayer Iron and Iron-Rich Hydroxide Nanosheets Exfoliated from High-Quality Green Rust for Enhanced Electrocatalytic Oxygen Evolution Reaction

“…As it is of critical importance to maintain the high crystallinity and phase purity of Fe(OH) 2 platelets, any transportation and characterizations under an ambient atmosphere must be reduced to the minimum level during subsequent treatments. A traditional way of transforming brucite-like Fe(OH) 2 into GR adopts aerial oxidation in an aqueous solution containing the desired interlayer anions (Cl − , SO 4 2− , Br − , etc.). 30 However, the resultant products always possess a gel-like form and nanoscale lateral dimension.…”

“…Transition metal (Ni, Co, Fe)-based layered double hydroxides (LDHs) are increasingly employed as promising electrocatalysts for the oxygen evolution reaction (OER) in water splitting under alkaline conditions due to their low cost and robust activity. − Especially, operando characterizations and computational studies demonstrate that iron plays an essential role when incorporated in M–Fe (Ni–Fe, Co–Fe) LDHs. − Boettcher and co-workers further revealed the OER activity trend of single metal (oxy)­hydroxides as FeO x H y > CoO x H y > NiO x H y , indicating the high potential of iron (oxy)­hydroxides as electrocatalysts . However, the poor electrical conductivity of bulk Fe (oxy)­hydroxides limits the electron transfer, resulting in a higher overpotential (η) than those of M–Fe (oxy)­hydroxides.…”

Heterostructure Engineering of 2D Superlattice Materials for Electrocatalysis

“…[ 166 ] For the 2D materials with weak interlayer bond force (e.g., graphene), [ 167 ] a top‐down approach is more suitable. [ 101 ] Single dispersed 2D monolayer nanosheets can be obtained by exfoliating the bulk materials, and then the corresponding superlattice materials can be obtained through layer‐by‐layer (LBL) assembly [ 139 , 151 , 157 , 164 , 168 , 169 ] or flocculation precipitation method. [ 81 , 82 , 99 , 100 , 101 , 104 , 105 , 110 , 140 , 145 , 146 , 149 , 150 , 155 , 170 ] For the 2D materials with strong interlayer bond force (e.g., TMDs), a bottom‐up approach can reach high‐quality 2D superlattice structures.…”

Boosting electrocatalytic water oxidation of NiFe layered double hydroxide via the synergy of 3d–4f electron interaction and citrate intercalation

“…20 Hu constructed NiOOH with a large structural disorder by increasing the Fe dopant and established a volcano-type correlation between the turnover frequency of Fe sites and structural disorder. 21 H. Goddard III and his workers also pointed out that d 4 Fe(IV) led to the formation of the active O radical intermediates, while d 6 Ni(IV) catalyzed the subsequent O-O coupling. 22 All these attempts lower the reaction energy barrier of the OER by optimizing the 3d-3d electron interaction between Ni and Fe atoms.…”

“…[3][4][5] To date, due to the combined merits of ideal adsorption/ desorption on intermediates and robust stability, noble metalbased materials, such as IrO 2 and RuO 2 , usually serve as benchmarks for industrial OER catalysts. [6][7][8] However, the deciency and valuableness greatly restrict their large-scale applications. On the other hand, non-precious transition-metalbased catalysts such as hydroxides, oxides, selenides, and phosphides [9][10][11][12][13][14][15][16] have gained much attention.…”