Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel–Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability

Abstract

Electrochemical oxygen reduction reaction (ORR) catalysts that have both high activities and long-term stabilities are needed for proton-exchange membrane fuel cells (PEMFCs) and metal-air batteries. Two-dimensional (2D) materials based on graphene have shown high catalytic activities, however, carbon-based materials result in significant catalyst degradation due to carbon oxidation that occurs at high electrochemical potentials. Here, we introduce the synthesis and electrochemical performance of metallic 2D nanoframes which represent a new approach to translate 2D materials into unsupported (carbon-free) electrocatalysts that have both significantly higher ORR catalytic activities and stabilities compared with conventional Pt/carbon electrocatalysts. Metallic Ni-Pt 2D nanoframes were synthesized by controlled thermal treatments of Pt-decorated Ni(OH) nanosheets. The nanoframes consist of a hierarchical 2D framework composed of a highly catalytically active Pt-Ni alloy phase with an interconnected solid and pore network that results in three-dimensional molecular accessibility. The inclusion of Ni within the Pt structure resulted in significantly smaller Pt lattice distances compared to those of Pt nanoparticles. On the basis of its unique local and extended structure, the ORR specific activity of Ni-Pt 2D nanoframes (5.8 mA cm) was an order of magnitude higher than Pt/carbon. In addition, accelerated stability testing at elevated potentials up to 1.3 V showed that the metallic Ni-Pt nanoframes exhibit significantly improved stability compared with Pt/carbon catalysts. The nanoarchitecture and local structure of metallic 2D nanoframes results in high combined specific activity and elevated potential stability. Analysis of the ORR electrochemical reaction kinetics on the Ni-Pt nanoframes supports that at low overpotentials the first electron transfer is the rate-determining step, and the reaction proceeds via a four electron reduction process. The ability to create metallic 2D structures with 3D molecular accessibility opens up new opportunities for the design of high activity and stability carbon-free catalyst nanoarchitectures for numerous electrocatalytic and catalytic applications.

Bimodal nanoporous platinum on sacrificial nanoporous copper for catalysis of the oxygen-reduction reaction

“…These values are significantly higher than those of the conventional Pt/C [17] and comparable with the values reported recently for other nanoporous Pt-based alloys. [1,[20][21][22][23][24][25] Furthermore, the copper dissolution increased the specific activity of the BNP-Pt. Above all, a higher activity per ECSA (and not per mass or geometric area) for BNP-Pt after copper dissolution means that the intrinsic catalytic nature of the platinum is enhanced by the disappearance of the supporting skeletal NP-Cu, although the overpotential itself is not so different between the two BNP-Pt samples.…”

Hydrous cobalt–iridium oxide two-dimensional nanoframes: insights into activity and stability of bimetallic acidic oxygen evolution electrocatalysts

“…33 Our group has developed bimetallic two-dimensional (2D) nanoframes that combine highly active bimetallic surface sites and a porous, carbon-free nanoarchitecture to provide oxygen electrocatalysts with improved activity and stability. 14,34,35 The 2D nanoframe structure is synthesized from thermal reduction of precious metal-decorated metal oxide nanosheets and is composed of a unique hierarchical 2D framework containing interconnected solid metallic alloy domains within a porous matrix that provides 3D molecular accessibility. We recently reported unsupported, hydrous iridium-nickel oxide 2D nanoframes exhibit 14 times higher OER mass activity than commercial IrO 2 .…”

Capacity and phase stability of metal-substituted α-Ni(OH)2 nanosheets in aqueous Ni–Zn batteries

“…The a-Ni(OH) 2 nanosheets were synthesized using a microwavesynthesis technique modified from our previously reported microwave-assisted hydrothermal method, 47,48 by adapting the protocol to a large-format microwave reactor at the Naval Research Laboratory (CEM Mars 5 Digestion Microwave System microwave reactor). In the modified synthesis, Ni(NO 3 ) 2 Á6H 2 O (3.0 g, Sigma-Aldrich) and urea (2.46 g, Sigma-Aldrich) were added to a solution of ethylene glycol (63 mL, Sigma-Aldrich) and ultrapure water (9 mL, Z18 MO cm; used for all subsequent reactions).…”

“…46 We recently reported that microwave-synthesized a-Ni(OH) 2 nanosheets serve as a high surface-area precursor to high-activity bimetallic OER electrocatalysts. 47,48 Microwave synthesis using neutral/mild pH conditions generates a-Ni(OH) 2 with a 3D flowerlike morphology 49 unlike the low surface-area aggregates characteristic of highly basic conditions. 44,45,50 Microwave synthesis has been used to synthesize a number of metal-substituted a-Ni(OH) 2 materials such as Mn/ Co-substituted a-Ni(OH) 2 as a precursor for Ni/Mn/Co oxide materials for supercapacitors, 51 3D flowerlike Co-substituted a-Ni(OH) 2 for supercapacitors, 52,53 and Al/Zn substituted a-Ni(OH) 2 .…”