Nanoporous carbons derived from binary carbides and their optimization for hydrogen storage

Improved sorbents with increased selectivity and permeability are needed to meet growing energy and environmental needs. New forms of carbon based sorbents have been discovered recently, including carbons produced by etching metals from metal carbides, known as carbide derived carbons (CDCs). A common method for the synthesis of CDC is by chlorination at elevated temperatures. The goal of this work is to synthesize CDC from ternary carbides and to explore the links between the initial carbide chemistry and structure with the resulting CDCs properties, including porosity. CDC was produced from MAX-phase carbides, in particular Ti₃SiC₂, Ti3AlC2, Ti₂AlC, and Ta2AlC. Additionally, CDC was produced from Ta-based binary carbides, TaC and Ta2C, and one carbo-nitride Ti₂AlC_[0.5]N_[0.5]. The CDC structure was characterized using XRD, Raman microspectroscopy, and HRTEM. Porosity characterization was performed using sorption analysis with both Ar and N2 as adsorbates. It was determined the microporosity of CDC is related to the density of the initial carbide. The layered structure of the MAX-phase carbides lent toward the formation of larger mesopores within the resulting CDCs, while the amount of mesopores was dependent on the chemistry of the carbide. Furthermore, CDC produced from carbides with extremely high theoretical porosity resulted in small specific surface areas due to a collapse of the carbon structure. To expand the potential applications for CDC beyond powder and bulk forms, CDC membranes were produced from a thin film of TiC deposited by magnetron sputtering onto porous ceramic substrates. The TiC thin film was subsequently chlorinated to produce a bilayer membrane with CDC as the active layer. Both gases and liquids are capable of passing the membrane. The membrane separates based on selective adsorption, rather than a size separation molecular sieving effect. Two applications for CDC produced from MAX-phases were investigated: protein adsorption and gas separation. Sorbents capable of adsorbing large protein molecules efficiently are desirable for many medical applications, including the treatment of sepsis. Primarily mesoporous Ti₂AlC-CDC and Ti3AlC2-CDC were proven to adsorb a significant amount of proteins compared to two current carbon adsorbents. When tested for gas separation, CDC was capable of selectively adsorbing gases including SF6, CO₂, CH4, and H2. However, the gases were not separated based on their size, but rather on their interaction with the CDC surface.

Porous solids are of great technological importance due to their ability to interact with gases and liquids not only at the surface, but throughout their bulk. Although large pores can be produced and well controlled in a variety of materials, nanopores in the range of 2 nm and below (micropores, according to IUPAC classification) are usually achieved only in carbons or zeolites. To date, major efforts in the field of porous materials have been directed towards control of the size, shape and uniformity of the pores. Here we demonstrate that porosity of carbide-derived carbons (CDCs) can be tuned with subångström accuracy in a wide range by controlling the chlorination temperature. CDC produced from Ti3SiC2 has a narrower pore-size distribution than single-wall carbon nanotubes or activated carbons; its pore-size distribution is comparable to that of zeolites. CDCs are produced at temperatures from 200-1,200 degrees C as a powder, a coating, a membrane or parts with near-final shapes, with or without mesopores. They can find applications in molecular sieves, gas storage, catalysts, adsorbents, battery electrodes, supercapacitors, water/air filters and medical devices.

Characterization and prediction of double-layer capacitance of nanoporous carbon materials using the Quantitative nano-Structure-Property Relationship approach based on experimentally determined porosity descriptors

Processing of carbide-derived carbon (CDC) using biomorphic porous titanium carbide ceramics

The oxygen reduction reaction (ORR) is one of the most important electrochemical reactions, used in metal-air type batteries and different types of fuel cells. The traditionally used catalyst, platinum, is expensive and scarcely available and thus many new catalysts have emerged, among which are heteroatom-doped carbon materials. Various carbon nanostructures have been widely used as base materials for heteroatom doping due to their high electrical conductivity, porosity and stability in electrochemical conditions. Owing to their porous structure, reactants and products are easily transferred to and from the active sites. Among these materials, carbide-derived carbon (CDC), which is a carbon material produced by removing metal atoms from a carbide lattice, stands out as an exceptional way to gain a base material with tunable properties. The specific surface area of CDC materials is typically in the range of 1000–2000 m2 g−1, the pore size is easily tunable via selection of different starting carbides and synthesis temperature with reproducible results in large scales. Previously, we have studied ORR electrocatalysis on undoped CDC materials in alkaline media. These materials catalyzed a 2e‒ reduction of O2.1 In this work, we demonstrate a pyrolysis method to synthesize nitrogen-doped carbide-derived carbon catalysts using titanium carbide as a base material and dicyandiamide (DCDA) as a nitrogen source. The CDC materials are obtained using chlorination to remove the titanium and then pyrolyzed along with DCDA at 800 °C for 2 h in a tube furnace to substitute nitrogen into the carbon lattice.2 The physical characteristics of the catalyst materials were studied using N2 physisorption, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy, which revealed that the materials had been successfully nitrogen doped, had a very high specific surface area and were highly amorphous. SEM revealed highly porous materials consisting of particles of various sizes (Figure 1a). These catalysts were then tested using the rotating disk electrode (RDE) method in alkaline solution toward the ORR. The mediocre activity of the CDC materials was increased by the nitrogen-doping procedure, which further increased the already high specific surface area of the materials and produced nitrogen-containing groups on the surface of the catalysts. The catalysts produced via the nitrogen doping of carbide-derived carbon are superior to the catalyst synthesized by chlorinating titanium carbonitride, but still not as active as 20% Pt/C (Figure 1b). However, the process for CDC synthesis is already used commercially and adding nitrogen doping to it would be just one more step. The N-doped CDC catalysts also showed excellent stability over 1000 potential cycles, which is an important property for fuel cell applications. Overall, the nitrogen-doped CDC catalysts are promising cathode materials for alkaline membrane fuel cell as they are stable and highly active toward ORR. Work is in progress to test these materials in fuel cell conditions. References I. Kruusenberg, J. Leis, M. Arulepp, and K. Tammeveski, J. Solid State Electrochem., 14, 1269 (2010). S. Ratso, I. Kruusenberg, M. Käärik, M. Kook, R. Saar, M. Pärs, J. Leis, and K. Tammeveski, Carbon, 113, 159 (2017). Figure 1

Methane adsorption on specially designed TiC and Mo2C derived carbons with different pore size and surface morphology

Electrochemical double-layer capacitors (EDLCs) store energy at the interface of the electrode and electrolyte, known as the electrical double layer. One of the challenges facing EDLC technology is their moderate energy density compared to batteries and fuel cells. Therefore, improving EDLC performance requires identification of the key electrode and electrolyte parameters that affect energy density. Some of the key parameters that are known to impact the energy density of an electrode material are the material’s specific surface area (SSA), its pore size distribution, the macroscopic morphology, and the material’s chemical composition, relating to the wettability of the material and the faradaic reactions that may occur during charging. However, published research to date has not isolated the intercoupled relationship between specific surface area and pore size distribution. While high surface area materials often have high microporosity (pores less than 2 nm in diameter) due to the high surface to volume ratio of these small pores, as the pore size is increased into the mesopores range (2-50 nm diameter pores), the material’s SSA tends to decrease, since mesopores have a lower surface to volume ratio [1, 2]. As such, there remains an ongoing unresolved debate as to the relationship between pore size and SSA-normalized capacitance [2], with some researchers demonstrating an increased SSA-normalized capacitance in pores with diameters less than 1 nm [3], some researchers suggesting that there is no correlation between pore size and SSA-normalized capacitance [4], and some researchers demonstrating that there is a relationship between SSA-normalized capacitance and pores greater than 2 nm in diameter [5]. One of the main challenges to unravel the correlation between pore size and SSA-normalized capacitance is the variation in a material’s structural features, including total SSA, and its chemical composition. This makes it challenging to determine the effect of a single parameter [2]. In order to investigate the effect of pore size on SSA-normalized capacitance, the ideal materials for comparison would be those of similar specific surface area, and chemical composition, but differing pore size distribution. This approach ensures that the effects of the material’s total SSA are constant. When materials have different total SSA values, it is difficult to determine if the variations in performance are associated to differing pore size or differing surface area. Additionally, the relationship is more complicated in a sample with multiple pore sizes, which is common in activated carbon materials. It is critical to determine the relationship between pore size and capacitance in materials with narrow pore size distributions, and materials that have broad pore size distributions. By identifying the key structural features that improve the energy density of a material, the overall performance of EDLCs may also be improved. This study investigates the relationship between SSA-normalized capacitance and pore size distribution. Activated carbon materials were prepared with various pore size, but with similar specific surface area and chemical composition. The relationship between pore size and both SSA-normalized capacitance and gravimetric capacitance as well as the rate performance of the activated carbon materials is reported. Finally, the dependence of capacitance on maximum operating voltage is discussed in order to better elucidate the relationship between porosity and capacitance. Initial results have demonstrated that as the pore size increases, the SSA-normalized capacitance decreases. However, a similar decrease was not observed in the gravimetric capacitance. This indicates that the variation in bulk density of broad pore size distribution carbon materials may mask the effects of enhanced SSA-normalized capacitance in sub-nanometer pores. The results suggest that there may be a critical pore size distribution that will maximize both SSA-normalized capacitance and gravimetric capacitance.

The mechanical properties of carbide-derived carbons (CDCs) are computed using molecular dynamics simulations, spanning the experimental density range and synthesis temperatures. The structures consist of nanoporous networks with continuous graphene walls enclosing the pores. Calculation of elastic constants and simulation of tensile strain reveal a direct relationship between the microstructure and elasticity, with the density and temperature inducing significant changes in the pore topology and medium-range order. CDCs have a high elastic moduli and high ultimate tensile strengths while showing resistance to brittle fracture. This suggests that CDCs are a promising route to achieve dense 3D graphene networks with tunable mechanical properties.

SiC-based fibers, eleven in total, derived from polycarbosilane pyrolysis and belonging to the three different generations, were fully converted into microporous carbon after selective etching of silicon atoms under pure chlorine flow at intermediate temperatures (550–850 °C). In this work, relationship between as-received fibers and their subsequent carbide-derived carbon (CDC) properties, pore structure oxidation resistance and mechanical properties, was investigated. The resulting carbon is microporous or micro-mesoporous with equivalent specific surface area exceeding 1000 m2 g−1 and pore size distribution (PSD) related to the substrate chemical composition and microstructure of former fibers. Oxidation kinetics were found to depend on this PSD. The former sp2 carbon free network of fibers remained unaffected by the chlorination. Its amount and percolation were respectively correlated to Young modulus and tensile strength of CDC. This carbon thus displays a nano-composite-like behavior with a former etched carbide matrix (transformed SiC and SiCO) and carbon free reinforcement. The results demonstrate CDC allows the control over pore size and mechanical properties by selecting an appropriate multiphasic substrate, combining etched and non-etched compounds.

Effect of lattice structure of silicon carbide on crystal formation of carbide-derived carbon

Micro- and mesoporous carbons are attractive electrode materials with the unique porous structure, narrow pore size distribution and possibility to fine-tune the pore size for electrodes of electrical double layer capacitors (EDLC), commonly called supercapacitors. The excellent adsorptivity of carbide-derived carbon (CDC) has been demonstrated but these materials are quite expensive and the main aim of this work was to establish conditions for the preparation of nanoporous carbon electrodes, based on less expensive raw materials, synthesis and activation (i.e. with H2O vapor) methods.

We investigate the effect of heat treatment on the structure of carbide-derived carbons (CDC) prepared by chlorination from nanosized βSiC particles and on their methane as well as hydrogen storage and delivery performance. Pore size and pore wall thickness distributions of the CDCs are obtained from interpretation of argon adsorption data using the finite wall thickness (FWT) model. The adequacy of the FWT model for adsorption modeling in the SiC-CDC samples is demonstrated by satisfactory prediction of subatmospheric and high pressure adsorption isotherms of CO2 and CH4 at 313 and 333 K. From the characterization results, it is observed that the SiC-CDC particles are predominantly amorphous with slight graphitization of the external surface. The degree of graphitization is more pronounced in the sample prepared at 1000 °C and increases slowly with heat treatment time. During this time the accessibility of methane molecules is found to increase, as a result of short-range ordering and opening up of pore ...

Pore characteristics of carbide-derived carbons obtained from carbides with different carbon volume fractions

Importance of pore size in high-pressure hydrogen storage by porous carbons

Because one of the promising solutions to the greenhouse effect is to develop sustainable energy, significant progress has been made in developing improved electrochemical storage systems. Supercapacitors, and more especially, electrochemical double layer capacitors (EDLCs) have attracted increasing attention over the past 10 years. However, EDLCs currently lack the energy density necessary to fulfil industrial demand, where most applications need high power for several tens of seconds. Today's research is focused on developing new carbon materials exhibiting higher specific energies. Important improvements have been obtained using nanoporous carbons (highly microporous, i.e. having a pore diameter below 2nm and even less). For this purpose different precursors have been extensively studied such as seaweeds, lignocellulosic materials and carbide-derived carbons. Nanoporous carbons seem to be the key to obtaining enhanced active materials since strong interactions with adsorbed ions leads to higher specific energies, because of, among other things, shortening of the electrode–ion distance together with the electrochemical double layer structuration. Future breakthroughs will be achieved by optimising and studying the electrode–electrolyte interface.