Research

Outline of Nishihara's group

Development of advanced functional carbon materials

It is difficult to precisely control the structure of carbon-based materials with non-crystalline frameworks. Moreover, precise structure drawing of such non-crystalline materials is also a difficult issue. We have developed the new techniques which allow the bottom-up synthesis of advanced carbon materials with controlled structures at atomic/molecular scale, specifically using organic synthesis or chemical-vapor deposition. Thus, a variety of functional carbon materials have been achieved such as metal-carbon frameworks with defined chemical structures like organic crystals, micro/mesoporous materials with singlegraphene walls, and carbon-based composite materials. Also, we focus on the elucidation of physicochemical properties of carbon materials including reactivity, durability, and catalysis from the view point of chemistry by using advanced analysis techniques. Moreover, we proceed in the application of our advanced carbon-based materials for supercapacitors, secondary batteries, fuel cells, heat pump, new energy devices, functional adsorbents, catalysis, and healthcare, with many collaborators including research organizations and companies.

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Research topics

There are many publications describing "graphene porous materials" and "three-dimensional (3D) graphene materials". However, most of them are materials with stacked graphene walls or materials with macropores rather than micro/mesopores. Our group is working on nanoporous materials with single-graphene walls, namely "single-walled nanoporous graphenes", such as zeolite-templated carbon (ZTC, Fig. 1) and graphene mesosponge (GMS, Fig. 2). These materials possess unique properties such as developed micro/mesopores, oxidation resistance, high conductivity, and mechanical flexibility, and can be distinguished from conventional nanoporous carbons and other graphene-based materials. Thus, they are expected to a variety of applications including supercapacitors, lithium-ion batteries, fuel cells, heat pump, and catalyst supports.

Fig. 1 Structure model of ZTC.

Fig. 2 Structure (left) and a TEM photo (right) of GMS.

Nanoporous materials with single-graphene walls are mechanically flexible, and they can be reversibly contracted and recovered like plastic sponges. Most of nanoporous materials are mechanically hard, and only a few types of nanoporous materials exhibit a large degree of deformation by mechanical force. We define such elastic nanoporous materials as 'nanosponges', and open the door of new physicochemistry based on the 'hyper nanospace' which can be deformed by mechanical force. For instance, it is possible to induce liquid-gas phase transition by squeezing nanosponges containing liquid (Fig. 3). Thus, the latent heat can be controlled by mechanical force, and this mechanism enables the design of a new type of heat pump.

Fig. 3 Liquid/gas phase transition using nanosponge.

Fig. 4 The cooling effect incused by sandwiched nanosponge.

Carbon materials are prepared by carbonization of organic precursors. During the carbonization process, the structure of a precursor is greatly changed, resulting in the formation of disordered and amorphous carbon structures. Recently, carbonization of crystalline materials like metal-organic frameworks (MOFs) has been intensively investigated, whereas only amorphous carbons have been synthesized. We have discovered a way to synthesize ordered carbonaceous frameworks (OCFs) which retain the structure regularity as well as molecular blocks of the precursor organic crystal (Fig. 5). OCFs are the hybrid of crystalline materials like MOFs and carbon materials, and we are developing new catalysts including alternatives for platinum catalysts.

Fig. 5 Synthesis scheme of OCFs.

We are developing healthcare applications using carbon-based materials. An example is flexible honeycomb monolith with micrometer channels. Honeycomb monoliths which are widely used for car mufflers are produced by extrusion molding, whereas the minimum channel size is restricted to about 200 micrometers. In 2004, Tamon, Mukai, and Nishihara developed a preparation method for honeycomb monoliths with channel sizes of 5 to 200 micrometers, via ice-templating approach. Moreover, Nishihara et al. succeeded in downsizing of channel size to 180 nm. In 2016, Nishihara et al. discovered a distinct structure-directing function of cellulose nanofibers for the microhoneycomb structures, and moreover, honeycomb monoliths with sponge-like flexibility were developed by compositing with graphene. We are developing a variety of healthcare applications using the honeycomb monoliths with micrometer straight channels and mechanical flexibility.

Fig. 6 Comparison of conventional honeycomb monolith and honeycomb monoliths with micrometer-scale channels.

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Highlighted research

2022: In-depth analysis of key factors affecting the catalysis of oxidized carbon blacks for cellulose hydrolysis (PDF)