NETL Researchers Convert Coal Tar Pitch Into Graphene for Improved Supercapacitors

NETL researchers have developed a low-cost process for converting coal tar waste into a high-quality graphene - material that can increase performance of energy-storing supercapacitor systems by up to 55%.

Supercapacitors store electrical energy and, in some ways, are comparable to batteries. One primary difference is that supercapacitors can be charged and discharged rapidly over the course of a few seconds or minutes, while batteries take hours. Supercapacitors can be used for up to 1 million charge/discharge cycles, but batteries can only be used for several thousand cycles.

Supercapacitors are often used to regulate power fluctuations between the electrical grid and intermittent renewable power sources such as windmill- and photovoltaic- farms. They are also used for short term energy storage as uninterruptable power sources. In many applications, supercapacitors and batteries are used together in hybrid energy storage systems where the strengths of each device offset the weaknesses of the other to create a synergistic system that stores and delivers energy across a range of conditions.

According to NETL's Christopher Matranga, one of the authors of the report published in the journal Small Methods, graphene has long been considered an ideal supercapacitor electrode material, but its use in commercial devices is limited because there are few methods for producing high-quality graphene at a large scale at a low cost.

Co-authors of the NETL report were Viet Hung Pham, Congjun Wang, Yuan Gao, Jennifer Weidman and Ki-Joong Kim.

"Graphene is one of the most promising supercapacitor electrode materials because of its large surface area, high electrical conductivity, good chemical stability, and excellent mechanical strength," Pham, the principal investigator on the report explained.

The NETL research discovered a process that uses coal tar pitch, an inexpensive and abundant carbon feedstock, along with a potassium carbonate (K2CO3) catalyst in a simple process to make microscopic 3D graphene with high carbon mass yields.

Coal tar pitch is a viscous liquid mixture of hydrocarbon compounds that is derived directly from coal and is frequently used as a binder or coating in the manufacture of lithium-ion battery electrodes.

The NETL report indicated that the new innovative process has produced high-quality graphene with capacitive properties that are among the highest ever reported for graphene-based supercapacitor devices.

Pham said that supercapacitors bridge a gap between conventional capacitors and traditional batteries by storing more energy than capacitors and possessing higher power densities than batteries.

According to the NETL report, the research results "overcome a long-standing challenge for fabricating practical supercapacitor electrodes, which involves balancing two conflicting characteristics: high porosity and high material density…As such, our simple synthesis method allows for the inexpensive production of electrodes and opens new opportunities for using graphene-based materials more broadly in practical supercapacitor devices."

NETL is a U.S. Department of Energy national laboratory that drives innovation and delivers solutions for a clean and secure energy future. By leveraging its highly skilled innovators and state-of-the-art research facilities, NETL is advancing carbon management and resource sustainability technologies to enable environmental sustainability for all Americans.

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