Microwave mode of heating in the preparation of porous carbon materials for adsorption and energy storage applications – An overview
Introduction
Tremendous industrial and research efforts are being done nowadays to find efficient ways to transform wastes and biomass into valuable products. Elaboration of carbonaceous materials, activated carbons (AC) in particular, figures among the most promising solutions. Owing to the ability to develop high specific surface area (more than 3000 m2 g−1), tuneable surface texture and functionality, ACs became an important class of porous materials with numerous applications in large-scale industrial processes such as water and air purification, solid/liquid extraction, gas-storage and separation [1,2]. These materials are also largely employed in electrochemical energy storage devices, medicine and catalysis [[1], [2], [3], [4], [5]].
Due to disordered character and local heterogeneity, the description of AC microstructure is difficult. Defective graphene-type sheets are considered as elementary building blocks of these carbon materials. Those are usually disordered, mainly due to oxidised edges and occurrence of 4-, 5- and 7- membered carbon atom cycles, lacunas and heteroatoms being at the origin of local distortions and corrugations breaking down the flatness and energetic homogeneity of the polyaromatic structures. Individual graphene-like layers are frequently stacked together over a very short lengths resulting in domains where interlayer spaces are supposed to create a microporosity. Voids between randomly arranged domains cross-linked together form, depending on their size, the super-microporosity and mesopores of ACs1 [[6], [7], [8]]. Mesopores in AC so originate from the arrangement of nm-sized domains or even individual particles composed of several domains as well as from the templating effect of naturally-occurring (mineral fraction) and introduced (template or activating agents) inclusions [9,10].
For several applications of ACs, the relationships between their performances and textural/structural properties are well established. Thus, for example, the presence of ultra-micropores in AC adsorbents is necessary for efficient separation of CO2 from CH4 under kinetic control in Pressure Swing Adsorption processes [11,12]. The prevalence of mesopores and high wettability are required for AC application for pollutant removal from aqueous media [2,13]. High specific surface area together with optimal pore size and high electric conductivity are fundamental for high-performance supercapacitors [14], whilst partial substitution of carbon by nitrogen allows carrying out electrochemical oxygen reduction reactions and hydrogen evolution reactions under overtensions comparable or even lower than those of platinum electrodes [15].
Textural and structural properties of ACs are determined by the nature of precursors, ways and conditions of their carbonization and activation but also post-synthesis treatment. To date the optimization of characteristics of these materials bears almost exclusively empirical character. Nevertheless, relying on well abundant experimental data, it is possible nowadays to select materials with properties designed for specific applications.
The diversity of AC precursors is broad, formally all raw materials can be divided into two groups: derived from fossil sources and bio-sourced. Numerous reviews and textbooks are available on this topic [[16], [17], [18], [19]], several examples are mentioned below. Preparation of porous carbon materials rely on thermochemical processes of pyrolysis and activation occurring at 300–1000 °C, accompanied by a series of complex reactions and transformations including depolymerization/re-polymerization, aromatization, decarboxylation, dehydration and gasification as well as morphological metamorphoses leading to the formation of carbonaceous bulk with highly developed porosity and surface area. Thereby, provided that available and abundant bio-sourced and organic-rich waste precursors are used for AC synthesis, the cost-effectiveness of large-scale production is conditioned by heating and sometimes washing (chemical activation) processes. It has been roughly estimated that on average, depending on precursors and synthesis conditions, 170-200 kilo-watt hour of electric energy is needed just to prepare 1 tonne of AC using traditional heating modes, with the environmental impact being equivalent to 127.5–150 kg of CO2 per tonne of AC. This is comparable to burning 160–188 kg of coal [20]. Because of growing demand in ACs for the traditional areas of application and the development of novel technologies involving these materials [21], in the context of actual energetic and environmental situation, the reduction of energy penalty and environmental impact of processes employed for AC production and regeneration is of prime importance. As microwave irradiation can be generated using clean and renewable electrical sources, in addition enabling very fast heating, this technology appears as an energy efficient alternative solution to prepare and regenerate ACs.
Industrial processes employing microwave irradiation are currently employed in food [22], rubber [23], sterilization, medical treatment [24], agricultural and forestry-related industries [25]. Because carbon materials and their organic precursors are usually good microwave absorbers [26], numerous research works have been carried out regarding microwave-assisted processes for their production, modification and regeneration. The main goal of the present work is to review the recent progress in microwave-assisted AC synthesis and regeneration with the emphasize on the influence of this heating mode on final product structure and properties in comparison to conventional synthesis and heating methods. Main advantages and limitations of the technology are revisited in order to better identify and position its up-scalability.
Section snippets
Technical aspects of microwave heating
Microwaves (MW) belong to the electromagnetic spectrum having a wavelength from 1 mm to 1 m [27]. Microwave energy is nonionizing electromagnetic radiation having frequencies in three different bands: ultra high frequency (300 MHz–3 GHz), super high frequency (3 GHz–30 GHz) and extremely high frequency (30 GHz–300 GHz) [28]. In order to avoid interferences with radar transmission or telecommunication [27], the use of MW for industrial, medical and domestic goals is conventionally regulated at
Pyrolysis under microwave irradiation
Pyrolysis is one of the key thermochemical processes used for converting biomass to valuable products. Carried out in anoxic conditions at temperatures ranging between 300 and 1300 °C, it results in formation of solid char (biochar), oil (bio-fuel) and gaseous (syngas) [27,41] products. It is convenient to distinct primary and secondary pyrolysis. Devolatilization accompanied by depolymerization, dehydration, oxidation and decarboxylation reactions are the main processes taking place in primary
Microwave-assisted activation of carbon
While in several cases chars may feature high-specific surface area [87,88], the activation step is frequently required to develop textural characteristics of carbons. Two main processes are usually employed: (1) physical activation – controlled partial gasification of char at 500-900 °C under CO2 or water steam atmosphere and (2) chemical activation – heating of char or raw material impregnated with an activating agent (KOH, ZnCl2, H3PO4 etc.), up to 600-1000 °C [4].
During physical activation,
Applications of microwave prepared AC
As introduced earlier, the applications of ACs rank among adsorption for gas and liquid purification, supports of catalysis, sensors and nanostructured electrodes for supercapacitors and LIBs. One of the most important properties that remains primarily assessed to characterize the potential of application of ACs is their specific surface area which is considered to be related with their adsorption or faradic capacity, or for their ability to support catalytic phase. The specific surface area of
Advantages and limits of microwave irradiation for AC preparation
Several studies in the literature have compared ACs prepared from a same biomass using either conventional or microwave heating modes [72,75,81,82,127,131,[145], [146], [147], [148]]. The following observations were made regarding the different preparation modes:
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AC can be produced with far shorter times under microwave heating, the required duration of the thermal activation is significantly reduced, to typically about a few minutes (10-15 min) when a conventional heating mode requires hours
Microwave-assisted heating technology in activated carbon regeneration
The exhaustion of adsorption properties of activated carbons is usually associated with the blockage of the nanosized pores with compounds to be removed from gaseous and liquid media such as volatile or soluble organic compounds in case of air treatment or water remediation or by “contaminants” preferentially interacting with adsorption sites such as H2S in case of CO2 removal from biogas by Pressure Swing Adsorption processes [[11], [12]]. The regeneration of spent activated carbons, aiming
Production cost of activated carbon using microwave and conventional techniques
The commercial price of activated carbon is 0.79–5 € per kg. The production cost of steam microwave activated palm shell is 0.65 € per kg [79]. The estimated production cost of microwave irradiated banana peel AC (NaOH–KOH activation) was 0.79 € per kg [81]. The estimated cost of microwave prepared AC with palm residue (NaOH–KOH activation) was around 3.53 € per kg for pilot-scale production and 9.01 € per kg for lab-scale production [73]. The microwave irradiated tannery-based sludge activated
Summary
In terms of quality and performance in different applications, AC synthesized from microwave technology stands parallel to the AC produced from conventional heating technology. Microwave heating stands for sustainable energy development by consuming lower quantity of resources, with low cost of production. Yet microwave-assisted AC preparation is not being adopted by industries because of the lack of accurate temperature measurement. Cheapest and accurate temperature measuring system certainly
Declaration of competing interest
None.
Acknowledgment
None.
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