**2. Materials for ANG storage**

The selection of a suitable adsorbent is one of the most important criteria for the utilization of the technology for successful commercial application. An adsorbent with high surface area, large pore volume, narrow micropore distribution, the pore size of 1–1.2 nm, and high density is required to achieve the ambitious ANG storage target for vehicular application set by the US DOE. The porous materials studied in recent literature for adsorption based natural gas storage at relatively low pressure compared to that of the CNG and ambient temperature conditions are resins, zeolites [5], xerogels [6], aerogels, carbon-based materials [7] such as carbon nanotubes and fibers, metal organic frameworks (MOF) and covalent organic frameworks (COF). The methane adsorption efficiency is known to be linearly dependent on the surface area of the adsorbent (**Figure 1**) [8]. Earlier studies have revealed that Zeolites exhibiting adsorption capacity up to 100 V/V storage capacity are not suitable to reach the target set by the DOE and the interest has shifted to the carbon-based materials and 3D frameworks [4].

*Adsorbed Natural Gas Storage for Vehicular Applications DOI: http://dx.doi.org/10.5772/intechopen.101216*

**Figure 1.**

*Schematics showing the effect of the surface of the carbon-based adsorbents on the methane uptake capacity, the red squares represent the carbon in granular powder form and the green circles represent the coal samples.*

The volumetric storage capacity of the adsorbed natural gas may be calculated in a simple manner by following the equation given below;

$$\mathbf{n}\_{\text{stg}} = \mathbf{n}\_{\text{exc}} + \mathfrak{p}\_{\text{gas}} \left[ \mathbf{1} - \left( \mathfrak{p}\_{\text{pack}} / \mathfrak{p}\_{\text{He}} \right) \right] \tag{1}$$

where, nstg is the volumetric storage capacity, nexc is the excess amount of adsorbate per volume of adsorbent, ρgas, ρpack, and ρHe is the gas, packing and helium density respectively.

### **2.1 Porous carbon materials**

Porous carbon materials are one of the widely studied systems for said application. In general carbon-based materials have exhibited higher adsorption capacity compared to that of the other studied porous materials possessing similar surface area due to their slit-shaped pore structure [9]. The packing density of activated carbon-based materials is also known to exhibit a linear dependence on the applied pressure and the value reaches up to 0.8 g/cm3 on applying pressure till 980 MPa [10]. Furthermore, the activated carbon-based materials can be easily synthesized from readily available low-cost starting materials such as wooden materials, corns, different fossil fuels, and polymer and are cost-effective. These materials can be easily physically activated using steam or CO2 and chemically activated in an industrial scale using acids and bases under high-temperature conditions [11]. Earlier studies on carbon-based adsorbent for the natural gas storage application was based on the single-walled carbon nanotubes (SWCNT). The Monte Carlo simulation-based theoretical studies predicted that the SWCNT bundles may be suitable for methane storage under moderate pressure conditions [12]. However, experimental studies afterward suggested that the adsorption capacity is limited to 0.11 g/g in these systems at 60 bar and 30°C [13]. The activated carbon-based materials have been successful in achieving excess gravimetric methane uptake till 0.2 g/g at 35 bar and ambient temperature conditions and in this case the delivery capacity was 170 V/V at 65 bar pressure [14]. Further increase in pressure to 100 bar improved the adsorption capacity to 263 V/V, which surpassed the target set by DOE. However, the deliverable capacity of carbon-based materials is reported to be inferior, which limits the utilization of these materials for storage applications. The carbon nanotube (CNT) though has shown promise to achieve high adsorption capacity, the experimental value is limited to 160 V/V at 35 bar and 25°C [15].

One of the advantages of these carbon-based materials is that these can be synthesized from renewable, polymeric, and cost-effective sources and activated via multiple routes. For example, olive stones were utilized to prepare carbon-based materials, which on carbonization at 500°C under inert atmosphere produced the carbon material. The sample was activated using KOH at a high temperature of 800°C [16]. The sample exhibited 3551 m2 /g BET surface area and 215 V/V methane uptake capacity at 100 bar with a working capacity of 135 V/V. In another instance, polyacrylonitrile beads were pyrolyzed at 600°C to synthesize carbon flowers. The carbon flowers were then activated with KOH by heating the mixture at 800°C. The flowers were also activated by heating these under CO2 flow at 850°C. The resulting carbon materials exhibited a BET surface area of 1077 m2 /g and methane adsorption capacity of 196 V/V at 65 bar [17]. Other biomass precursors for the synthesis of porous carbon materials include coconut shells [18], corn straws [19], banana peels [20], and soya [21]. Xiao and coworkers published a summary on the use of various biomass for the synthesis of porous carbon materials and the surface area of the resulting materials [22]. The review also summarized the different activation methods utilized to activate the carbon materials such as strong inorganic bases [23], lewis acid [24], and H3PO4 [25] based procedures to optimize the pore structure further and enhance their adsorption performance. Templating is another useful technique utilized in literature to achieve highly ordered and large surface area carbon materials. Both soft and hard templating can be used for this purpose. Hard templates could be a various nanoparticles, silica, and molecular sieves. For example, mesoporous silica sieve hard templates in presence of 1,10-phenanthroline ligand were utilized to synthesize Co immobilized nitrogen-doped porous carbon materials for catalytic applications. The calcination for the material synthesis was carried out at 800°C [26]. Typically, in the case of the soft templating method, the polymers or surfactant self-assemble into a particular ordered shape, which is then immobilized into the mesoporous material to be synthesized. Subsequently, the templates are removed to expose the pores and obtain the porous material. Ionic liquid and self-assembled block polymers have been utilized as the soft templates for this purpose [27, 28]. Direct carbonization of ordered nanostructures is also becoming another attractive option to generate porous carbon materials. For example, the MOF can be utilized as precursors to directly synthesize porous carbon materials via pyrolysis under an inert environment [29]. The presence of various organic ligands may also serve as the carbon source for the above synthesis, whereas the metal nodes often help to control the pore structure and control the physical properties of the carbon material. For example, Yamaguchi and coworkers utilized a Zn and Co-based bimetallic MOF to synthesize nanoporous carbon by pyrolyzing the precursor under an inert atmosphere at 900°C [30]. The shape of the precursor was replicated in the carbon material produced as can be shown in **Figure 2** below. Metal such as Zn evaporates at a high temperature allowing the formation of pores in the resulting carbon product. In some cases, the metal ions convert to the corresponding nanoparticles and serve as a catalytic site for further applications. The type of metal, the metal content, and ligand type play important role in developing nanoporous carbon structures.
