Effect of isobutane adsorption on the electrical resistivity of activated carbon fiber cloth with select physical and chemical properties

Abstract

It is important to develop new materials to improve our ability to purify gas streams. This study characterizes activated carbon fiber cloth’s (ACFC’s) electrical resistivity and the change in resistivity from isobutane adsorption. Results are presented for ACFC samples with select ranges of micropore volumes (0.35–0.92 cm³/g) and surface oxygen contents (2.2–7.7 at.%). The resistivity of all samples decreased with increasing temperature, which is attributed to the additional energy for electron hopping between nanographite domains. Increasing micropore volume resulted in a larger decrease in resistivity after an isobutane adsorption cycle, which is attributed to increased adsorption capacity with increasing micropore volume. However, increasing surface oxygen increased the change in resistivity/unit mass of isobutane adsorbed. Increasing oxygen for lower oxygen content samples increased the percent change in resistivity due to isobutane adsorption, while increasing oxygen for moderate oxygenated samples had no effect on this value. Thus, increasing surface oxygen improves ACFC’s ability to sense adsorbed mass based on resistivity up to a threshold oxygen value. These results provide valuable insight into the factors affecting ACFC resistivity due to adsorption, which can be used to detect the amount of adsorbed mass and to determine the remaining adsorption capacity of the adsorbent.

Introduction

Activated carbon fiber cloth (ACFC) is an effective gas adsorbent and has unique electrical properties due to its local nanographitic structure. ACFC is comprised of a disordered 3-D nanographitic structure of nanographite domains, each consisting of 3–4 stacked nanographene sheets of 250–350 carbon atoms that are arranged similar to an aromatic polycyclic molecule, as shown in Fig. 1 [1]. The nanographite domains have mean in-plane dimensions of 2–3 nm and typically have an interlayer spacing between nanographene sheets of 0.38 nm [2], [3], [4], [5]. These nanographite domains are bound to each other in a disordered manner resulting in nanopores between domains that provide large surface areas (up to 2600 m²/g) for the adsorption of gaseous species [6].

The relationship between ACFC’s temperature and resistivity closely follows the Coulomb-gap variable range hopping model for granular metallic materials, in which electrons hop between the highly conductive nanographite domains [7], [8], [9]. For this model, the probability of electron hopping increases with increasing temperature due to added energy. Electron transport by hopping is dependent on the dimensions of and distance between the metallic grains (i.e., nanographite domains for ACFC) as well as the concentration of localized electrons within the grains. The dimensions and distance between ACFC’s nanographite domains can be readily altered through heat treatment [10]. The resistivity of ACFC is also largely affected by electron hopping that occurs at the edges of the nanographite domains [11]. The edges of the nanographene sheets (single sheet of the nanographite domain) are either comprised of energetically stable armchair edges or zigzag edges with localized nonbonding π-electrons [11]. The localized nonbonding π-electrons provide ACFC with active sites for surface functional groups or physical adsorption [11]. Thus, the electrical resistivity of ACFC is particularly interesting because it can be affected by heat treatment, surface functional groups, and physical adsorption as described below.

Heat treatment of a carbon-based precursor can affect the resistivity of the material by altering its physical and chemical structure. Moderate heat treatment of carbon (e.g., 450–900 °C) in CO₂ or steam alters the size and alignment of the nanographite domains, resulting in increased porosity [12]. This increased porosity decreases the cross section of the sample for current flow and increases the distance electrons travel to pass through the material and the distance for electron hopping between domains, resulting in increased resistivity [13], [14]. Heat treatment to higher temperatures (e.g., 2000 °C) graphitizes the carbon by increasing alignment of the nanographite domains, resulting in a more ordered nanographitic structure with lower resistivity [10], [15]. Thus, the temperature of the heat treatment affects the spatial configuration of the nanographite domains and the resistivity of ACFC.

Adding surface functional groups to ACFC can result in charge transfer that affects ACFC’s electrical resistivity. For example, doping of the ACFC’s surface with electron acceptors or electron donors decreases or increases the concentration of localized electrons in the ACFC, respectively. Oxygen is a particularly important electron acceptor because the edges of nanographene sheets readily react with oxygen resulting in oxygen functional groups, such as carboxyl, carbonyl, and phenol groups, in which charge transfer occurs from the nanographene sheets to the oxygen groups [1], [16]. For example, oxygen functional groups can be added to ACFC with HNO₃ treatments [13], [17]. These surface oxygen functional groups accept ACFC electrons, reducing electron hopping between the nanographite domains [1], [18]. By contrast, H₂ treatment is used to remove oxygen functional groups from ACFC, decreasing ACFC resistivity [13]. Thus, ACFC resistivity can be adjusted by altering the amount of surface oxygen functional groups on the sample (e.g., with H₂ or HNO₃ treatment).

Physical adsorption through van der Waals interactions alters ACFC’s physical structure, which also affects ACFC’s electrical resistivity. The interlayer spacing between the nanographene sheets of as-prepared ACFC is typically 0.38 nm, while the corresponding spacing for graphite is 0.335 nm, because edge effects create a less stable structure that allows these sheets to spread [5]. X-ray diffraction (XRD) and electron spin resonance studies have revealed that select physically adsorbed molecules reduced the interlayer spacing between the nanographene sheets, thus increasing the distance between nanographite domains and increasing electrical resistivity [3], [5], [19], [20]. This compression of the nanographite domains is particularly pronounced during water adsorption onto activated carbon fibers, in which the interlayer spacing decreases from 0.38 to 0.34 nm (determined with XRD), within 1.5% of the interlayer spacing for graphite [5]. Another XRD study revealed that the physical adsorption of benzene onto cellulosic activated carbon fibers increased the interlayer spacing between the nanographene sheets, suggesting attraction between the benzene and nanographene sheet [5]. Other organic molecules without hydroxyl groups decreased the resistivity of activated carbon, which is likely due to alterations of the nanographitic structure of the ACFC but additional studies would strengthen this research area.

To summarize, heat treatment, surface functional groups, and physical adsorption each affect the electrical resistivity of ACFC. Heat treatment affects electrical resistivity by altering the alignment and distance between nanographite domains, which affects the distance for electron travel and electron hopping. Adding surface functional groups to ACFC affects its resistivity by altering the amount of localized electrons. Physical adsorption affects resistivity by altering the distance for electron hopping between nanographitic domains.

Because ACFC’s electrical resistivity is influenced by adsorbed species, ACFC is a desirable material for environmental sensing applications. For example, the physical adsorption of hydrocarbons such as benzene, propane, butane, and isobutane decrease the resistivity of activated carbon rods [21], [22]. The reversible change in ACFC resistivity that occurs during physical adsorption allows ACFC to be used as a gas sensor. For example, measured ACFC resistance values have been used to determine the amount of mass that is adsorbed to ACFC to determine when it is saturated or regenerated [23], [24], [25]. This sensing and control technique is particularly attractive for manufacturing applications that exhaust a single volatile organic compound (VOC) in air that can be adsorbed onto ACFC to reduce the environmental and health impacts from emitting this VOC. For example, isobutane (i.e., a common refrigerant and propellant that is a VOC without a hydroxyl group) is used as a propellant during packaging manufacturing resulting in an exhaust gas stream containing isobutane in air that reacts in the atmosphere to form photochemical smog [26], [27]. Isobutane from this exhaust gas stream can be adsorbed onto ACFC to reduce its emission to the atmosphere [25]. It is desirable to be able to detect the amount of isobutane that is adsorbed onto the ACFC based on ACFC resistivity so that the exhaust gas stream can be redirected to regenerated ACFC before isobutane breakthrough to reduce emissions of isobutane to the atmosphere.

This study characterizes the electrical resistivity of ACFC samples to better understand the effects of ACFCs with select physical and chemical properties (i.e., micropore volume from 0.35 to 0.92 cm³/g and surface oxygen content from 2.2 to 7.7 at.%, respectively) on ACFC’s resistivity due to physical adsorption. The resistivity of each of the ACFC samples was characterized during the physical adsorption of isobutane. The pore structure, chemical structure, and nanographitic structure of the ACFC samples were also characterized. These research results are valuable because they describe the effects of micropore volume and surface oxygen content on the electrical resistivity of ACFC before and during isobutane adsorption and explains these effects based on analysis of the nanographitic structure and properties of the ACFC, allowing for improved selection of an adsorbent with desirable electrical properties for sensing adsorbed mass. These results can be used to improve the design of an ACFC gas sensor (by considering micropore volume, surface oxygen content, and adsorbate composition) such that the mass of select molecules that are physically adsorbed to ACFC can be detected based on electrical resistivity values.