Novel methods to fabricate macroporous 3D carbon scaffolds and ordered surface mesopores on carbon filaments
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- Lu, X., Chen, L., Amini, N. et al. J Porous Mater (2012) 19: 529. doi:10.1007/s10934-011-9501-x
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New methods for fabrication of 3D macroporous carbon scaffolds and synthesis of mesopores on carbon surfaces are proposed. Ordered macroporous filamentary carbon structures were made by rapid prototyping using solvent-based extrusion freeforming which allows the scaffold to be designed on computer and downloaded directly to a building platform. The surface of extruded filaments was decorated with 20–25 nm open mesopores by coating with nano-silica as a hard template followed by pyrolysis and dissolution of the silica. This left an open mesoporous surface to serve as a host for catalysts or enzymes while retaining integrity in the core for electrical and mechanical performance. The combination of these two methods could be used to make different hierarchical, multi-functional carbon structures which could be applied in fuel cells as the catalyst carrier and biofuel cell electrode.
KeywordsMesoporesCarbonSilica colloidFilamentary macroporous structure
Intricate special multifunctional carbon structures with a hierarchy of different pore distributions, each level of which serves a different purpose, are desirable for catalyst supports in reactors involving convective flow , for fuel cell electrodes  and in the development of biofuel cells  where they combine the roles of catalyst carrier, electrode and current carrier. Hierarchical porous carbon structures are therefore a goal for several research groups . The sequence of operations involves construction of carbon scaffolds with different shapes, creating mesopores with different sizes and immobilization of catalysts or enzymes. The first two steps are addressed here.
In order to improve the mass and heat transfer and to control liquid or gas flow directions, spatial arrangements involving macropores of complex design are required and the procedure described here allows such designs to be prototyped. Traditionally, macroporous scaffolds have been fabricated by hard-template  or foaming methods . Both are complex and the latter presents the difficulty of controlling the pore size and pore size distribution. In this paper, we propose the extrusion freeforming method to make 3D carbon structures. It makes use of a rapid prototyping method which is also capable of being used for manufacturing. The structure is designed on computer and downloaded directly to the building platform where it can deliver filaments down to 80 μm diameter  with inter-filament pore sizes down to ~5 μm. Extruded filaments of a paste consisting of polymer, fine particles (e.g. ceramic, metal) and solvent are assembled to construct nominally 2D or 3D structures. The solvent is propan-2-ol, sometimes used as a hand wash in the hospital environment. Extrusion freeforming has been successfully used for fabrication of EBG crystals  and artificial bone scaffold . Compared with similar technologies, such as fused deposition  and gel extrusion , paste extrusion is a relatively simple technique without heating, cooling or polymerization processes to contend with and can be easily controlled. This technology offers a new method to fabricate electrode and catalyst/biocatalyst carrier scaffolds.
In order to immobilize an enzyme or catalyst on the carrier surface, a particular mesopore size range is required. For example, biofuel cell enzymes require supports with pore size around 15–35 nm . However, the mesopores in the structure can be divided into two categories: accessible and inaccessible. Chemical catalytic reactions tend to take place on catalysts within multiphase fluids so the effective enzyme and catalyst hosts are mesopores situated on the accessible surface of the skeleton. If such pores are embedded within the structure, they are useless and can cause deterioration of mechanical and electrical properties. So a structural carbon supporting a mesoporous carbon film is the ideal electrode and catalyst host.
Structures of ordered porous carbon with one mode of mesopore size (normally 2–50 nm) have been reported. They are generally synthesized by nanocasting template  or self-assembly  methods. The ordering and spatial distribution are strongly dependent on the hard/soft templates. At present, there are difficulties in creating 20–25 nm surface mesopores. Traditional but effective soft-template methods using PVB (poly(vinyl butyral)) , Pluronic F127 (PEO106–PPO70–PEO106)  and FDU (Fudan University) mesostructures  which are suitable for preparation of well-ordered mesoporous carbon films can only make pores less than 10 nm. Other traditional methods using hard-templates like SBA-15 , MCM-48 , CMK-1  are not suitable for coating to create mesopores on the substrate surface. In this paper, we propose the ordered macroporous carbon structure fabricated by extrusion freeforming and a new method for creating mesopores on the surface of carbon filament. The combination of these two methods could develop more complex hierarchical porous carbon structures.
2 Experimental details
2.1 Fabrication of ordered filamentary macroporous carbon
Ordered macroporous carbon lattice structures were fabricated by solvent-based paste extrusion freeforming in which the extruded structure solidifies by transition of polymer from liquid to gel through solvent evaporation accompanied by an overall increase in solids content. Two different kinds of paste were tested. The first was based on formulations previously used for ceramics . The thermoplastic binder, poly(vinyl butyral) (PVB) (grade BN18, Whacker Chemicals, UK), and plasticizer, poly(ethylene glycol) (PEG) (MWt = 600, VWR, UK) were used at 75% wt. PVB, 25% wt. Graphite (GS6E, GrafTech International Ltd, UK) was incorporated at 60 vol.% based on the dry paste. A second paste was prepared from PVB, Novolac phenolic resin (Vilosyn 455, Vil Resins Ltd, UK) with a curing agent of hexamethylenetetramine (HMTA) (Reagent Plus 99%, Sigma-Aldrich, UK) with a mass ratio of PVB to resin 3:2 and graphite powder at 30 vol.% based on graphite + polymer + resin. In both cases, the solvent was propan-2-ol (General Purpose Reagent, VWR International, UK). In preparation, first PVB + PEG or PVB and resin + HMTA (w:w = 100:13.5) were dissolved independently in the solvent and then mixed. The graphite powder was added to the organic solution and dispersed by ultrasonic probe (U200S, IKA Labortechnik, Germany). The pastes were prepared by stirring the suspension manually while evaporating excess solvent to develop a paste suitable for extrusion. The viscosity of the paste was measured on a Haake cone and plate Reostress 150 rheometer.
2.2 Synthesis of surface mesopores on extruded carbon filaments
The novel method for the production of 20–25 nm mesopores on the carbon filament surface is based on a coating of colloidal silica (Fig. 1c) applied before pyrolysis. The as-extruded carbon paste lattices were dipped in the colloidal silica suspension (Ludox AS-40, 40 wt% silica, size: 22 nm, Sigma-Aldrich, UK) to deposit a thin surface film and then dried at 50–60 °C. in air. Because PVB and resin are not soluble in water, the filaments are not damaged by this step. This process was repeated up to four times but these thicker coatings tended to spall during drying. In order to prevent spalling, PEG (Mwt: 10,000, VWR, UK) was added to the colloidal silica. The filaments were fully dried in an oven and the temperature was increased to 140 °C for 6 h to cure the resin. For carbonization, the filaments were placed in a metal wall furnace and nitrogen (oxygen-free, BOC, UK) was used as the protection gas. The temperature was increased from room temperature to 400 °C at 2 °C/min with a dwell at 400 °C for 1 h, then from 400 to 800 °C at 5 °C/min with a dwell at 800° for 0.5 h. Finally silica was leached using 5 M NaOH solution (NaOH: 97%, Sigma-Aldrich, UK) for 7 days at ambient temperature.
3 Results and discussion
3.1 Ordered filamentary macroporous carbon structures
The solid freeforming stage depends on the flow properties of the graphite paste and the way viscosity changes as evaporation of solvent proceeds. Conventional extrusion of graphite pastes has been used to make electrodes. Baldrianova et al.  used a graphite paste based on highly viscous silicone oil mixed with graphite powder at a volume fraction of about 56% in disposable micropipette tips to make electrodes within the diameter range from 50 to 300 μm. Cervini et al.  used graphite powder mixed with polyurethane resin to extrude 3.0 mm graphite rods. But these types of paste do not satisfy the requirements for extrusion freeforming because the extruded filaments need to bend 90° to stack layer by layer and then weld at junction points to the preceding layer. Paste extrusion freeforming depends on an effective state change brought about by the polymer transition from liquid to gel through solvent evaporation accompanied by an overall increase in solids content which takes place after deformation and welding.
The advantages of ordered macroporous carbon fabrication by paste extrusion are: (1) Complex macroporous arrangements can be designed exactly according to the requirements of the porous carbon application and then constructed by the nozzle trajectory which is set by computer so that fluid flow and flow direction can be engineered into the final product; (2) No moulds or templates are needed; (3) The fabrication method is relatively simple, rapid and requires no heating, cooling or cross-linking. Various ceramic lattices designed for different scaffolds have been demonstrated and illustrate the scope of this procedure .
3.2 Surface mesopores on carbon filaments
Two transport mechanisms account for the final surface topography as seen in Fig. 7b. The main process takes place in the early stages of heating as the resin softens but before curing is completed and consists of capillary flow into the interstices of the assembly of nanospheres. During the subsequent heating and pyrolysis stages, infiltration of the voids between silica beads can also occur through the formation of pyrocarbons starting by nucleation and growth on the template silica beads . The SEM images use high magnification close to the limit for this instrument (JSM 6301, JOEL, Japan). The original silica beads deposited from colloidal silica and dried are shown in Fig. 1c in order to gauge their dimensions for subsequent microscopy.
The distinguishing features of this method are: (1) The precursors of carbon come from the internal filament which is covered by a silica bead layer. (2) The direction of carbon deposition proceeds from internal to external. It prevents the carbon covering the silica beads and is therefore favorable for silica leaching. (3) The mesopores formed on the filament are open to the outside so it is easier to immobilize enzymes and catalysts and is therefore suitable to chemical reactions. If the carbon deposition direction was from external to internal, it would be difficult to leach the silica beads because they become embedded and protected by carbon.
The next stage in the development of these structure for fuel cells, which is a focus of a continued study within the UK Biological Fuel Cells Consortium, involves the controlled modification of the carbon surface e.g. by nitric acid  or ammonia  followed by immobilization of the enzyme (such as laccase) on the functionalized surface. The adsorption can be assessed by FTIR to obtain information about the immobilized enzyme and for comparison with those of free enzyme . The activities of immobilized laccase can be evaluated by using ABTS in distilled water as the reaction substrate and one activity unit is defined as the amount of enzyme required to catalyze 1 μmol of substrate per minute . During the reaction process, the increase in the absorbance at 420 nm can be measured by a UV–vis spectrophotometer . Finally, the laccase-immobilized graphite electrode biofuel cell can be studied by cyclic voltammetry using a potentiostat . This work has been conducted for similar modified carbons intended for fuel cell applications .
The use of extrusion freeforming makes it possible to build 3D carbon structures with hierarchical pore arrangements. The macropore structure can be designed on the computer to fulfill convective flow conditions specified, for example, by computational fluid dynamics modeling. Intricate carbon structures with a hierarchy of pore size distributions each level of which serves a different purpose could have applications in fuel cells and biofuel cells as the catalyst carriers and enzyme-immobilized electrode. Biofuel cell enzymes require pore sizes 15–35 nm so a structural carbon with mesoporous surface carbon film is the ideal electrode and catalyst host. Open surface mesopores in the required range were produced using a hard template coating but some problems still need to be resolved. The mesoporous skin of the filament can separate into islands during drying and pyrolysis. The mechanical strength of the filament needs to be improved and these problems are being addressed.
The authors are grateful for the support of the Leverhulme Trust under grant number F/07476/V and the EPSRC under the SUPERGEN Initiative via. the UK Biological Fuel Cells Consortium (Grant no. EP/H019480/1).