Benign species-tuned biomass carbonization to nano-layered graphite for EMI filtering and greener energy storage functions

Highlights

• Nano-layered graphite single crystals from carbonized bamboo.

• Novel bio-graphite nanomaterials with surprisingly high conductivity.

• Groundbreaking bio-graphite applications in EMI filtering, batteries and sensors.

• Graphitic Bamboo composites ideal for thermal management applications.

Abstract

For the first time the electrical conductivity of bamboo biographite-based material reported a ground-breaking milestone of 4.4 × 10⁴ (S/m). This reported conductivity by far exceeded all previous reported conductivity measurements obtained from renewable carbon. Controlled high-temperature thermal carbonization of biomass, notably Asian bamboo, at extended residence times elicited surprising growth of nano-layered biographitic structures with a layer-to-layer distance of less than 0.3440 nm. Moreover, thermodynamically dispersed bamboo and pine biographitic nano-layered carbon-based lightweight composites in a polyamide matrix were found to be intrinsically conductive both thermally and electrically. Electromagnetic interference (EMI) shielding device made from bamboo renewable carbon/cellulose nanofiber (CNF) composites possesses EMI shielding effectiveness (SE) of ∼23 dB. These results constitute a new advancement in the materials science of nano-layered graphites from renewables and their applications as EMI filtering devices and as electrode materials in air cathodes, electronics, supercapacitors in energy storage devices, and thermal management of batteries and sensors.

Introduction

Renewable materials and renewable energy (consumable and non-consumable energy) matter are two key resources reported to be essential ingredients in the development of human civilizations since at least 4000 B.C. For example, human use of bamboo leaves and stems as shield and hunting tool components can be traced back to well before 3000 B.C., above and beyond their use as energy sources. As the global population approaches 8 billion, the demand for energy and materials will increase at an alarming rate. In order to mitigate climate change and protect the “eco-environment”, transformative renewables technologies will be needed to replace the use of fossil fuels. Charcoal (a carbonized version of biomass), for example, has been used for non-heating purposes since as early as 30,000 B.C [1]. Based on this ancient knowledge, we have worked for the past decade to investigate new devices and energy materials produced from a modified form of charcoal derived from bamboo and woody biomass. We have previously reported that bamboo and wood fibres form layered-graphite materials when treated at relatively high temperatures (∼> 600 °C), and that their graphite characteristics increase with temperature [[2], [3], [4]] and residence time. The layered-graphitic structure results from the increased number of graphene layers in the charred lignocellulosic residues. The many beneficial characteristics of graphite arise from graphene’s overlapping sp² hybrid bonds (where 1 s and two p orbitals hybridize and pi electrons delocalize freely [5]. Graphene is optically transparent and, moreover, the pi bonds provide it with high charge carrier mobility, high thermal conductivity, extreme stiffness, and gas impermeability [6,7]. There can also be an economic benefit to the substitution of bamboo- and wood-based graphite derived from low-cost natural fibres in place of commercially available graphite.

For example, such materials also have potential for use in air cathode batteries [8] and their resistance could be < 10 Ω. Air cathodes are an attractive green electrochemical technology in view of their simple design and potential to use oxygen directly from the air as an electron acceptor. Conductive renewable carbons also offer another promising application as EMI shielding materials in various electronic devices currently in widespread use [9], as well as microwave wireless communications (Zhao et al., 2019). The electromagnetic shielding effect contributes not only to the minimization of device malfunction but also to the promotion of a healthy living environment by absorbing and shielding electromagnetic waves (Zhao et al., 2019). Including layered-graphite materials in composites may potentially result in raising the composite conductivity by several orders of magnitude due to the formation of initial conducting channels in the polymer matrix. Electrical conductivity gradually increases as additional conductive pathways are created with the addition of conducting filler content until a saturation plateau is reached [10]. The applications of electrically conductive composites vary based on their electrical resistivity and electrically tunable properties. Such composites can be employed as conductors, sensors, anti-static materials, and electromagnetic interference shielding. For example, conducting composites with electrical resistivity of ∼10⁴ Ωm could be used for plastic fuel tanks, while those with electrical resistivity of ∼10⁻⁴ Ωm could be applied for electromagnetic interference [[10], [11]]. Additionally, the nanocomposites can also be employed as ultrafiltration membranes [12] and high-efficiency adsorbents [13] that could contribute to many water and wastewater treatments, thus further indicating the multifunctional applications of renewable carbon.

Among the conventional conductive composite fabrication methods are the melt mixing technologies of internal mixing, twin-screw extrusion, and injection molding [10]. The enhancement of composites’ electrical and mechanical characteristics may be achieved by: (i) improving the matrix and nanofiller mechanical interlocking and adhesion; (ii) setting the testing temperature above the glass transition to enhance the mobility of nanofillers within the polymeric matrices; and (iii) altering the crystalline phase of the composite, caused by the nanofiller’s addition [14]. The filler’s homogeneous dispersion without aggregates and the filler-matrix strong bonding allow efficient stress transfer as well as evading failure points during the testing procedures [15], suggesting that the dispersion of nanofillers into the polymeric matrices and the degree of nanofiller and polymer interactions greatly influence composite functional properties [16,17].

Creating thermodynamic compatibility between the polymer and the filler requires a critical balance of their surface energies. A thermodynamically compatible multi-component polymeric composition is an essential step to obtaining composites with desired functional properties. Maleated copolymers, such as maleated polypropylene (MAPP) and maleated polyethylene (MAPE), are known to create a surface energy balance with hydrophobic matrices, and hydrophilic fillers are thus used as hydrophobic-lyophilic thermodynamic balancers in composite application. The anhydride groups on a maleate-grafted polymer interact with the hydroxyl groups of natural fibres [18,19]. These copolymers are also observed to improve mechanical properties and work optimally when applied at ∼3% of the total composite weight [20].

Polyamide (PA) polymers have a number of industrial applications including but not limited to various automotive products, fibres, films, and textiles. These low-density thermoplastic engineering polymers contain repeating ̶ CONH (amide linkage) units that equip them with a high melting point and good mechanical properties as well as resistance to corrosion and abrasion [14], resulting in excellent performance to cost ratios. However, PA polymers have limitations in their physical characteristics including high moisture absorption, relatively low impact strength, poor notch sensitivity, and poor dimensional stability [21]. Therefore, improving these physical characteristics in polymer materials and adding various fillers in composites to introduce new properties have been of great interest in recent studies [14].

Here we explore a novel and potentially transformative approach to incorporating renewable carbonaceous materials to form nano-layered graphitic structures through the use of long-residence-time biochars. The impact of thermal exposure on electrical conductivity of the carbon materials is also analyzed. In addition, we examine diverse functional attributes of graphitic renewable carbons and investigate their doping effect in a polyamide matrix. This is achieved through examining the electrical conductivity, thermal conductivity, and mechanical properties of the composites.