Waste Biomass-Derived Carbon Materials for Energy Storage

By Samudrapom DamReviewed by Susha Cheriyedath, M.Sc.Oct 4 2022

A paper recently published in the journal ACS Applied Energy Materials demonstrated the feasibility of using a simple and effective method to exfoliate waste biomass into porous carbon with different structural levels for energy storage.

Study: Exfoliating Waste Biomass into Porous Carbon with Multi-Structural Levels for Dual Energy Storage. Image Credit: Billion Photos/Shutterstock.com

Background

Waste biomass-derived carbon-based materials have been used extensively in lithium-ion batteries (LIBs) and supercapacitors in recent years owing to their renewability, ease of access, good mechanical performance, and abundant storage.

Hard carbon derived from biomass possesses several advantages, including low expansion rate and electrode working potential, high capacity, and good cycling stability. Several hard carbons derived from biomass, such as walnut shells and reeds, have demonstrated exceptional electrochemical performance. 

Two-dimensional (2D) porous carbon nanosheets demonstrate a higher capacity compared to graphite, a common material used in LIB anodes, due to the increased layer spacing in them that allows greater storage of lithium ions and disorganized microcrystal stacking.

For instance, graphene-like nitrogen-doped 2D carbon nanosheets synthesized after palm spathe activation by potassium hydroxide (KOH) attained 477 mA h g⁻¹ capacity at 0.1 A g⁻¹ and 1297 m² g⁻¹ specific surface area (SSA).

Among the porous carbon materials, the open three-dimensional (3D) porous structure possesses high SSA, several quadratic active sites, low impedance, and a large electrolyte/electrode contact interface, which can considerably increase the energy density. Moreover, disordered ions can efficiently travel through this structure under high loading conditions.

Inexpensive biomasses with abundant heteroatoms and natural porous structures can act as precursors for functional carbon materials. Nitrogen/oxygen (N/O) in these porous carbons can enhance the electrochemical properties by improving the conductivity of materials and Faraday reaction pseudocapacitance.

Additionally, the formation of nitrogen-carbon (N-C) bonds can increase the contact area by regulating the electron domination of carbon, which improves the wettability between the electrolyte and porous carbon nanosheet surface.

Among the pore-making reagents traditionally used in chemical methods to prepare porous carbon, potassium hydroxide is the most efficient reagent to achieve high SSAs and fabricate different pore sizes. However, KOH causes instrument damage as it is susceptible to moisture and highly corrosive in nature, which necessitated the developed KOH-free waste biomass activation strategy.

The Study

In this study, researchers proposed using a one-step carbonization activation of waste biomass/peanut oil residues with non-toxic and eco-friendly potassium carbonate (K₂CO₃) to fabricate 2D porous carbon nanosheets and N/O self-doping 3D cage-like porous structures.

Peanut oil residue was initially crushed into powder and then dried for 12 h at 80 ^oC. The treated powder was then mixed with K₂CO_3, and the resultant mixture was ground and transferred to a nickel crucible.

The crucible was heated up to 900, 800, and 700 ^oC temperatures at a 5 ^oC/min heating rate under a nitrogen atmosphere for two h in a tube furnace. Subsequently, deionized water and two M hydrochloric acid were used to clean the porous carbons (PKs) to obtain a neutral filtrate.

The samples were then dried for 12 h at 80 ^oC and designated as PK-900, PK-800, and PK-700, respectively, depending on their respective carbonization activation temperature.

Field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy were performed for the characterization of the synthesized PK samples.

Nitrogen adsorption−desorption isotherm was performed to determine the pore size and surface area of all samples. A three-electrode system consisting of a counter electrode, a reference electrode, and a working electrode was used for supercapacitor testing.

The working electrode consisted of polytetrafloroethylene (PTFE) as a binder, acetylene black as a conductive material, and an activated carbon material dissolved in ethanol at a 1:1:8 ratio. Mercury/mercury oxide and platinum sheet were used as reference and counter electrodes, respectively.

Two electrolytes, including one M tetraethylammonium tetrafluoroborate/propylene carbonate (TEMATFB/PC) and six M KOH, were used to assess the working electrode during the supercapacitor testing.

Researchers used a CHI660E electrochemical workstation to measure the galvanostatic charge−discharge (GCD) and cyclic voltammetry curves in six M KOH. They calculated the gravimetric specific capacitances of electrodes using the GCD results during the supercapacitor testing.

During the LIB testing, the working electrode contained an activated carbon material, polyvinylidene fluoride, and acetylene black dissolved in N-methylpyrrolidone at an 8:1:1 ratio.

Observations

Porous carbon structures, including N/O self-doping 3D cage-like porous structures and 2D porous carbon nanosheets, were prepared successfully using the one-step carbonization activation of waste peanut oil residues/biomass precursor with K₂CO₃. This method represented an effective KOH-free biomass precursor activation strategy.

The peanut oil residue exfoliation by K₂CO₃ was gradually enhanced, and heteroatomic N/O self-doping was maintained at a specific amount by regulating the carbonization activation temperature, which led to the formation of N/O double-doped, pore-rich, and high-SSA carbon materials.

The synthesized porous carbon materials possessed a larger SSA in the range of 1657−2920 m²g⁻¹, a greater degree of graphitization, and a higher nitrogen doping at 1.38−4.41% and oxygen doping at 2.48−4.98%. The porous carbon maintained a high graphitization degree even when the disordered sp²-hybridized carbon increased, resulting in good conductivity.

The fabricated PK-800 sample possessed a 3D cage-like porous structure. During supercapacitor testing, the working electrode containing PK-800 as an activated carbon material achieved an exceptional area-specific capacitance of 4400 mF cm⁻² at 0.5 A g⁻¹ at high loads of 12.5 mg cm⁻², which confirmed the scalability and stability of this peanut oil residue-derived carbon structure.

Moreover, the symmetric components of PK-800 showed an excellent energy density of 60.31 W h kg⁻¹ at 372 W kg⁻¹ in the one M TEMATFB/PC electrolyte. The 3D porous structure retained up to 95% of its capacity at high loadings when used in the three-electrode supercapacitor in a six M KOH electrolyte.

The synthesized highly graphitized 2D porous carbon nanosheets/PK-900 demonstrated a high reversible capacity of 580 mA h g⁻¹ at 0.1 A g⁻¹ after 200 cycles when used in LIBs as the larger layer spacing structure efficiently stored the lithium ions.

To summarize, the findings of this study demonstrated an energy-efficient, multi-purpose, and eco-friendly strategy to activate waste biomass through one-step carbonization and exploit it in a layer-by-layer manner to prepare porous carbon at several structural levels for energy storage.

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