Rhodium nanoparticles anchored on 3D metal organic framework-graphene hybrid architectures for high-performance electrocatalysts toward methanol oxidation

This study is led by Dr. Haiyan He (College of Mechanics and Materials, Hohai University,China). Besides the exploration of new energy restheirces, developing economic and environmentally friendly energy storage and conversion devices is one of the efficient tactics to deal with the energy crisis. Among numerous functional devices, direct methanol fuel cells (DMFCs) have gained extensive traction due to many distinctive advantages including high energy utilization efficiency, simple cell structure, convenient transportation of liquid fuel, and low pollutant discharge. As one of the key components in DMFCs, the anode catalyst can help not only reduce the activation barrier but also speed up the methanol oxidation reaction. Currently, commercial catalysts are mainly platinum-based catalysts and their derivatives owing to their remarkable methanol oxidation reactivity. Hotheyver, the instability and inactivation of metal Pt particles have greatly hindered the catalytic capability owing to abundant factors, such as poisoning with intermediates, particles migration or agglomeration, metal oxidation and redeposition, which sparked the development of novel anode catalysts with high efficiency and long life-span. Recently, plentiful research interests have extended to other species in Pt group elements, such as palladium (Pd), rhodium (Rh), and iridium (Ir)-based series. Among them, recent experimental and theoretical researches have demonstrated that Rh catalysts are potential substitutes for Pt catalysts with respect to their competitive catalytic activity and superior resistance to CO species especially in the alkaline conditions.

Additionally, it is also an effective strategy to select suitable supports to improve the dispersion of noble metal particles, which can expose more catalytically active sites and simultaneously decrease the dosage of noble metals. In recent years, the emergence of various advanced carbon-based materials has inspired the adoption of catalyst matrixes, such as porous carbons, carbon nanotubes, and graphene. Generally, graphene with a single layer of sp²-hybridized carbon exhibits a large specific surface area, excellent conductivity, and reliable electrochemical stability. Whilst the chemical inertness of graphene surface retards the corrosion of carbonaceous substrates, it nevertheless lacks adequate active centers for loading metal particles. Furthermore, due to the van der Waals interaction of the graphene layers, the restacking and aggregation of two-dimensional (2D) lamellae usually result in a reduction of reactive sites. Within this context, the construction of 3D porous architectures can effectively prevent the graphene nanosheets from restacking and aggregation and meanwhile accelerate electrolyte transportation. Their recent studies have also shown that the 3D graphene frameworks can serve as multiple growth platforms for immobilizing small-sized noble metal nanocrystals, thus providing new design opportunities for the development of highly-active anode catalysts for DMFCs.

On the other hand, metal-organic frameworks (MOFs) are known as porous networks with rich functional groups and tunable pore sizes, which possess potential applications in the electrocatalytic fields. Among zeolitic imidazolate frameworks (ZIFs) with both zeolite and MOFs features, ZIF-8 is a type of rich-nitrogen skeleton with solid and ordered hierarchical porous structures along with large specific surface area, which can be adopted as the precursors of porous carbonaceous materials or metallic oxides after pyrolysis to further applications in the fuel cells. In addition, the large presence of N element in ZIF-8 is able to facilitate the preferential nucleation of noble metal nanocrystals to ensure their uniform dispersion, and simultaneously the abundant Zn component in ZIF-8 may serve as a co-catalyst or promoter for the electrocatalytic reactions. However, the limited conductivity and mechanical stability of MOFs restrict their direct use as anode catalysts of DMFCs. With this context, the combination of MOFs with a conductive matrix is an efficient way to enhance electrochemical performance. Therefore, it is of great significance to explore the possibility of 3D MOF-modified graphene hybrid architectures, which may supply rigid frameworks with adequate active sites for growth of metal nanoparticles and plenty of channels to shorten the diffusion pathways of electrolyte ions. If that can be accomplished, then it may enable to keep each individual advantages of each component in the hybrid system and activate the synergistic catalytic effects for fast reaction kinetics.

In this work, they have proposed a bottom-up strategy for the design and fabrication of ultrafine Rh nanoparticles anchored on a 3D hybrid architecture constructed from graphene nanosheets and zeolitic imidazole framework (3D Rh/G-ZIF) via a solvothermal self-assembly process. As shown in Scheme, the functional groups on ZIF-8 nanocrystals and GO nanosheets would be connected with each other under the solvothermal condition, thus giving rise to a 3D ZIF-8 modified graphene hydrogel. With the further introduction of the Rh(NO₃)₃ solution, the gradual growth of Rh nanoparticles on the surface of 3D G-ZIF frameworks could be achieved, resulting in the generation of the desired 3D Rh/G-ZIF hybrid architectures. By virtue of their unique structural advantages as they’ll as strong synergetic coupling effects, the prepared 3D Rh/G-ZIF catalysts show large electrochemical active surface areas, high mass activity, and reliable long-term stability toward the methanol oxidation reaction, which are apparently superior to those of conventional Rh/carbon black (Rh/C), Rh/carbon nanotubes (Rh/CNT) and Rh/graphene (Rh/G) catalysts.

In summary, they have fabricated ultrafine Rh nanoparticles decorated on 3D interconnected architectures built from graphene and ZIF-8 by a facile solvothermal self-assembly process. In this newly-designed hybrid system, the 3D hierarchical porous G-ZIF networks can facilitate the transportation of external electrolytes into the internal surfaces and simultaneously ensure high electrical conductivity, while the high dispersion of small-sized Rh nanocrystals with optimized electronic structures provides numerous catalytically active sites. As a result, the 3D Rh/G-ZIF architecture with an appropriate G/ZIF-8 proportion (1:5) possesses a large ECSA value, high mass, and specific activities, and reliable long-term durability toward the methanol oxidation reaction, far outperforming those of traditional Rh/C, Rh/G and Rh/CNT catalysts. The study provides a novel design strategy to build 3D stable and crosslinked metal-decorated MOF/graphene nanostructures, which have a wide range of applications in future energy conversion and storage areas.

See the article:

Rhodium nanoparticles anchored on 3D metal organic framework-graphene hybrid architectures for high-performance electrocatalysts toward methanol oxidation

https://doi.org/10.1016/j.asems.2022.100029