Preliminary investigation of electrical conductivity of monolithic biochar

Abstract

Monolithic biochar is explored as electrode material in supercapacitor – a fast-charging, long-lasting energy storage device. Electrical conductivity of electrode is critical to supercapacitor's performance. Traditionally, biochar is used as a solid fuel for combustion where electrical conductivity is largely irrelevant and ignored. This work explores electrical conductivity of monolithic biochar and elucidates its dependence on micro and macro structures of biochar. Electrical conductivity of biochar was found to be highly dependent on its degree of carbonization. Bulk conductivity of biochar can increase by over six orders of magnitudes when its carbon content changes from 86.8 to 93.7 wt%. Transmission electronic microscope and x-ray diffraction analyses revealed the presence of graphite nanocrystals (∼3 nm) and the growth of biochar crystallinity after heat treatment at 950 °C. The highest skeletal conductivity of carbon was 343.2 S/m, found in a heat-treated sugar maple biochar with 96.2 wt% of carbon. It is higher than that of graphite single crystal in direction perpendicular to graphene plane (333.3 S/m). Moreover, it was observed conductivity of monolithic biochar increased with compressive loading and dropped to the pre-compression level when the loading was released - a new phenomenon termed “elastic behavior of electrical conductivity of biochar”.

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.