Activated Carbon

Activated carbon is used to remove unwanted chemicals from gases, liquids, and mixtures, and for energy storage. Its applications are widely varied from shoe insoles to remove foot odor to use in food, beverage, and pharmaceutical purification. The primary modes of action for removing these unwanted chemicals is through physical adsorption onto the surface of the activated carbon (physisorption) or through chemical adsorption where chemicals bond to the surface (chemisorption). Physical adsorption is a reversible process and depends primarily on the total surface area (TSA) of the activated carbon. Chemical adsorption is not easily reversible and depends primarily on the availability of active surface area (ASA) on the carbon surface. Active surface area is primarily made up of edges of carbon atoms on the surface or chemically active chemicals impregnated into the carbon surface.

The conventional method to activating biochar is a lengthy and energy-intensive process. The activation requires chemical treatments at high temperatures (600-900°C) using steam or a strong base (e.g. KOH) for over two hours. Chemical activation is effective in creating micropores of ~2 nm size and subsequently achieving large surface areas >1,000 m2/g. However, the surface area is not the only factor that determines the performance of activated carbon. The accessibility of the adsorbates to the porous surfaces is critical.

The following figure represents a porous network in a carbon granule, which includes micropores (<2 nm width), mesopores (2–50 nm), and macropores (>50 nm). Macropores and large mesopores facilitate transporting the adsorbates, whereas a large number of micropores lead to large surface area to achieve high adsorption capacity.

Our research has confirmed that the pore size distribution in chemically activated biochar falls into a narrow range, which limits the mobility of the adsorbates. Furthermore, the thermal chemical treatment encounters fundamental limitations in functionalizing biochar surfaces, which are necessary in many applications. For instance, the currently used activated carbon is inefficient in removing some types of PFAS and becomes progressively less effective in removing shorter chain PFAS.

Activated Carbon Uses

Purification

Activated carbon is commonly used as the primary media in filters to purify gases, liquids, and mixtures. These filters can be both passive, where the filters adsorb the unwanted chemical onto the surface as they pass over the carbon surface, and active, where the unwanted chemical bonds onto the active sites. Additionally, a passive surface may be activated by applying an electrical current to the carbon to attract electrically charged contaminants from the media to the carbon.

This is commonly called capacitive deionization (CDI) and is most commonly used in water purification. Two applications of CDI are described below:

PFAS Removal: An emerging focus of the water purification industry is the need to remove a human carcinogenic family of chemicals from ground and surface water This family of chemicals is commonly referred to as PFAS and pertains to the family of per- and poly-fluoroalkyl chemicals that have been widely used in consumer products and industry applications. Examples of PFAS products include non-stick cookware, food packaging materials, and firefighting foam. The EPA health advisory level for PFAS chemicals is a total of 70 parts per trillion (ppt).

Widespread use and extreme resistance to degradation have resulted in the ubiquitous presence of these compounds in the environment. Using activated carbon to adsorb PFAS in contaminated water is currently the most practical and mature technology. Activated carbons have shown the ability to consistently remove PFAS at parts per billion or micrograms per liter (𝜇g/L) concentrations with an efficiency of more than 90 percent. Since PFAS compounds are polar (electrically charged), using CDI or activated carbons with high surface capacitance can improve the overall removal efficiency of filtration systems.

Desalination: Removing salts and other ionic chemicals from water is another important application for activated carbon. For this application, CDI is the primary method of using activated carbon and carbons with high surface capacitance are the best for this application. While desalinating sea water is attractive in arid parts of the world, treating brackish water is particularly important in many agricultural regions in the US. CDI, with high surface capacitance carbons, is especially important due to its high efficiency (up to 99%) and low energy consumption (~$0.11 m-3 for capacitive deionization vs. $0.35 m-3 for reverse osmosis).

Energy Storage

Supercapacitors (electric double layer capacitors) are extremely useful for energy storage applications where high energy output is needed for fractions of seconds. Uses for supercapacitors include electric grid stabilization, varied automobile applications, and power supplies. Key attributes of supercapacitors include high power, fast charge-discharge, and a long life cycle.

A typical supercapacitor formed into a coin cell. The coin cell components include a metal foil surface, storage media, electrolyte, and charge separator.

The key component within a supercapacitor is the charge storage media which must have good electrical conductivity, electrochemical stability, abundantly available, and easily manufactured. Various carbons, such as carbon nanotubes, aerogels, and graphene, meet these requirements and are emerging as media of choice. Activated carbon is emerging as an alternative to higher cost carbon media and biochar based activated carbon has shown the ability to produce higher surface capacitance than many other forms of activated carbon.