Preliminary investigation of electrical conductivity of monolithic biochar
Graphical abstract
Introduction
Lignocellulosic biomass is plant-based material which captures and stores atmospheric carbon dioxide via photosynthesis. This biomass is an emerging source of carbon-neutral fuel [1]. Dry lignocellulosic biomass, such as wood and agricultural wastes, typically consists of 50 wt% of carbon. Carbon in biomass can be retained via carbonization or pyrolysis by which biochar, a carbon-rich product, is manufactured [2], [3]. Pyrolysis processes can be slow or fast depending on the feed and the desired quality of biochar. Microwave pyrolysis is new way used to produce biochar [4], [5], [6], [7], [8]. Biochar structures range from graphite-like carbon to high molecular weight poly-aromatic hydrocarbon rings [9], [10], [11]. Biochar is inherently porous due to its precursor structure. Highly carbonized biochar (>90 wt% of carbon) is chemically stable under ambient conditions. Production and utilization of chemically stable biochar can be considered as a mechanism of carbon capture and storage – an overall carbon-negative process that removes carbon from the carbon cycle. In one application, biochars have been shown to improve soil fertility and crop performance by balancing the pH of acidic soils [3] and increasing microbial activity [12]. In another application, owning to its chemical stability and electrical conductivity, biochar has been explored as electrode materials in energy storage and in electrokinetic soil remediation [2].
More specifically, traditional methods to store electrical energy include batteries and dielectric capacitors [13]. Electrochemical double layer capacitors (EDLCs), or supercapacitors, are energy storage devices that have properties shared by both batteries and capacitors. Batteries can store high energy but charge or discharge slowly, whereas capacitors can charge nearly instantaneously but have less storage capacity. Supercapacitors. In this work we focus on the electrical conductivity properties for supercapacitors. In EDLC, electrical conductivity of its electrode material is critical to its performance and electrodes have high electrical conductivity and high surface area. In general, conductivity is a measure of the rate of charge transfer through the device. Carbon electrode materials currently under study include graphene and carbon nanotubes which possess high electrical conductivity, high surface area and high mechanical strength accentuated with good cycling stability [14], [15], [16], [17]. However, these materials face many challenges associated with economics and scale-up.
Although there has been some biochar conductivity data in literature, it is limited to powdered porous biochars [18], [19], [20], [21]. This work focuses specifically on the electrical conductivity of monolithic biochar from wood – a topic that has not been systematically explored. The goal is to reveal its structure and property relationship to answer the question about what controls the conductivity of biochar. The new knowledge gained from this study will guide the design and manufacture of highly-conductive porous biochar for energy and environmental applications.
Section snippets
Biochar sample preparation
Biochar samples for conductivity measurements were prepared from commercial charcoal products obtained from three different suppliers. As expected, commercial charcoal products were highly variable in their appearance and carbon content. Given the heterogeneous nature of commercial products, charcoal pieces were screened and selected using a visual inspection. The focus was the blackness of the sample and the continuity of the surface, i.e., no visible cracks. The biochar was produced
Effect of degree of carbonization
Dry biomass typically contains 50 wt% of carbon. Carbonization or pyrolysis drives out hydrogen as well as oxygen and results in an increase in carbon content of the carbonized samples (i.e. biochar). Carbon content in biochar is therefore the measure of degree of carbonization. Table 1 displays the results of elemental analysis of the commercial biochar samples selected for this study. Carbon contents of these samples varied from below 75 wt% to above 95 wt%, suggesting a wide range of degree
Conclusions
A method was developed to measure electrical conductivity of monolithic biochar, while eliminating contact resistances and maintaining the structural integrity of samples. This method enabled this first systematic study on a fundamental property that is particularly important to applications where biochar is used as the electrode material, such as supercapacitor.
Electrical conductivity of biochar is highly dependent on its degree of carbonization which is measured by its carbon content. Higher
Acknowledgment
The authors acknowledge Natural Science and Engineering Research Council (NSERC) of Canada for funding this research.
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