Hydrothermal Synthesis of a rGO Nanosheet Enwrapped NiFe Nanoalloy for Superior Electrocatalytic Oxygen Evolution Reactions

Abstract

Graphene-based hybrid nanostructures possess many advantages in the field of electrochemical energy applications. In this work, a facile and efficient hydrothermal approach has been developed for the preparation of NiFe alloy nanoparticles/rGO hybrid nanostructures, in which the nanoparticles are well combined with rGO nanosheets and the size of the nanoparticles is about 100 nm. Moreover, the electrochemical oxygen evolution reaction (OER) tests confirmed that the obtained NiFe/rGO hybrid nanostructures possess notably higher activity than both the rGO-free NiFe nanoparticles and pure Ni/rGO hybrids, and the optimal NiFe ratio is 2:1. The OER overpotential at 20 mA cm(-1-2) with Ni2 Fe/rGO is as low as 0.285 V, which is 96 mV lower than that of pure Ni/rGO hybrids. Meanwhile, the Ni2 Fe/rGO catalyst has excellent stability. Therefore, this work contributes a facile and efficient method to prepare a NiFe alloy nanoparticles/rGO hybrid structure for potential applications in the field of electrochemical energy devices, such as electrochemical water splitting cells, rechargeable metal/air batteries, etc.

NiFeCo oxide as an efficient and sustainable catalyst for the oxygen evolution reaction

“…The overpotentials at various current densities are shown in Figure C for each catalyst. The lowest overpotential (248 mV at 1 mA cm −2 ) obtained from the ternary NiFeCo oxide electrode is comparable to the values reported for complex binary and ternary compound electrodes such as NiFe‐LDH/rGO catalyst (296 mV), NiFe (OH)x/FeS/IF (245 mV), Fe 0.2 Ni 1.8 OOH/EGSI (237 mV), Ni 2 Fe/rGO is as low as 285 mV, 2D NiFe‐based metal‐organic framework (310 mV), NiCoFe layered triple hydroxides (239 mV), FeCoNi (288 mV), and trimetallic NiFeMo (238 mV) . For fair comparison, we provided additional FeCo and NiCo metal oxide data as Supporting Information.…”

Tannic Acid-Mediated In Situ Controlled Assembly of NiFe Alloy Nanoparticles on Pristine Graphene as a Superior Oxygen Evolution Catalyst

“…36,37 In addition, there are not enough surface functional groups on GO for the in situ assembly of NiFe alloy NPs since strong reductants conventionally used for the reduction will make the nucleation so fast that they easily grow in solution rather than on the support. 30,40 To date, it remains elusive to fabricate pristine graphene supported small and well-dispersed NiFe alloy NPs, and it is even more challenging to controllably assemble these NPs on the pristine graphene. To our knowledge, no one has reported the controlled synthesis of NiFe alloy NPs supported on pristine graphene, and no one has employed them as OER catalysts.…”

“…One strategy to improve intrinsic catalytic activities of transition metal-based OER catalysts is the synthesis of mixed transition metal-based materials, which could outperform their single-component counterparts. − The NiFe-based catalysts are particularly promising since both Ni and Fe are earth-abundant transition metals, and addition of an appropriate amount of Fe into Ni-based catalysts could greatly increase their OER activities. − Recent studies have shown that various NiFe-based compounds, including their oxides and oxyhydroxides, and their hybrids formed by adding other components, can show high OER catalytic activities. − However, their performance still needs significant improvement due to their low conductivity and small catalytically active surface area. The NiFe alloys could be an attractive option to address these challenges in terms of tunable content of Fe and excellent conductivity; in particular, the catalytically active sites of NiFe oxides/(oxy)­hydroxides could be derived from their surfaces spontaneously both in the ambient environment and during the anodic oxidation. ,, Until now, there have been only a few studies using NiFe alloys as OER catalysts, and the reported ones still have relatively low activities. ,, Several great challenges exist for significantly improving their OER activities: (1) NiFe alloy NPs with greatly reduced size have significantly increased surface area (researchers are even reducing the size of OER catalysts to the single-atom level to increase the number of exposed active sites), but they easily aggregate to reduce the accessible active sites on them and inhibit the mass diffusion; , (2) the electron transfer/transport between these alloys and the electrode is slow and needs to be greatly accelerated; , (3) both strong reducing agents and large surfactants are required to create the non-noble NiFe alloy NPs, making the synthesis process nonenvironmentally friendly and complicated; , (4) approaches to controllably synthesize NiFe alloy NPs for optimizing their OER activities are still lacking. , …”

“…With a π-conjugated two-dimensional (2-D) structure and ultrahigh conductivity and surface area, graphene could be promising as a support for the assembly of small NiFe alloy NPs with high dispersion and prevention of their aggregation during storage and electrochemical measurements, while creating fast electron-conducting paths, thus significantly improving the OER catalytic performance. − However, pristine graphene nanosheets are very inert due to their scarce functional groups and 2-D π-conjugated structure, and therefore, it is rather difficult to assemble NiFe alloy nanostructures on them without destroying their chemical and electronic structures. , This will not only promote the aggregation between nanostructures during both the preparation and testing processes but also lead to restacked graphene sheets, thus reducing the accessible surface area of the catalysts. , Graphene oxide (GO)/strongly oxidized graphene could serve as the support due to its relatively rich functional groups, but it would compromise the performance even after complicated reduction processes using strong reductants since the electronic stucture has been damaged irreversibly leading to 2–3 orders of magnitude lower electronic conductivity of chemically reduced GO than that of pristine graphene. , In addition, there are not enough surface functional groups on GO for the in situ assembly of NiFe alloy NPs since strong reductants conventionally used for the reduction will make the nucleation so fast that they easily grow in solution rather than on the support. , To date, it remains elusive to fabricate pristine graphene supported small and well-dispersed NiFe alloy NPs, and it is even more challenging to controllably assemble these NPs on the pristine graphene. To our knowledge, no one has reported the controlled synthesis of NiFe alloy NPs supported on pristine graphene, and no one has employed them as OER catalysts.…”

Recent Progress on NiFe‐Based Electrocatalysts for Alkaline Oxygen Evolution

“…At the same time, the OER performance can be efficiently excited by the changed electronic structure and the optimal adsorption strength of reaction intermediates. [ 43,44 ] For example, Wang’s group formulated a route to synthesize core/shell NiFe nanoalloy containing N‐doped graphite shell (Figure 2c), [ 45 ] which required an overpotential of 320 mV to reach 10 mA cm −2 in 1.0 m KOH. The synthetic discrete graphite shells play a role in limiting the NiFe alloy core and generating more active sites.…”