Functionalization of Spent Coffee Grounds-derived Carbon Materials for Hydrogen Evolution Reaction and Water Purification

Abstract

Global coffee production has reached 178 million coffee bags, which consist of 60 kg in 2022, producing massive amounts of organic waste. One ton of coffee product generates about 650 kg of coffee waste, known as spent coffee grounds (SCG), through the coffee brewing process and instant coffee production. It equals over 6.8 million tons of waste, including fine-sized particles and high humidity, organic load, and acidity. Therefore, sustainable treatment and management solutions are urgently needed to reduce the environmental pollution of SCG and other bio-wastes. As reported in the literature, coffee waste can be turned into valuable products and utilized in many applications, such as adsorbents, electrocatalysts, fuel cells, supercapacitors, and batteries. Biowaste-derived activated carbon is the greener pathway to solve pollution issues. Due to its high specific surface area and porosity, it is sustainable, cost-effective, and has massive potential for use in various applications. However, activated carbon itself has low catalytic activity, and it needs to be functionalized with heteroatoms, metals, and those combinations to improve conductivity and catalytic activity. Currently, the adsorbent for water purification and electrocatalyst for hydrogen evolution reaction (HER) are crucial topics among researchers due to water pollution from many industries and the vast global demand for energy sources. Non-precious metallic electrocatalysts are attracting attention for use in the hydrogen evolution reaction (HER) because they are inexpensive, highly efficient, and durable. Notably, carbon-based catalysts, such as transition metal and heteroatomic doped carbon materials, are promising candidates to replace precious metal catalysts. Functionalizing the carbon matrix with nitrogen and cobalt is an effective way to develop the electrocatalytic properties of carbon-based catalysts. However, simple and cheaper catalyst synthesis methods have not been thoroughly investigated. Moreover, ruthenium (Ru) catalysts have a great potential to replace bench market catalysts (platinum) in electrocatalyst applications due to similar hydrogen bond strength and relatively lower prices. Here, heavy metal-free carbon materials were prepared from spent coffee grounds (SCG) by using KOH-urea and NaOH-urea as activating agents, and these were compared with SCG activation by the alkali salt alone. The SCG was impregnated with the activating agents and pyrolyzed at 800°C under a nitrogen (N2) atmosphere. The surface morphology structures and chemical components of the as-pyrolyzed carbon materials were examined by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Raman spectroscopy X-ray photoelectron spectroscopy (XPS), and measurement of nitrogen adsorption-desorption isotherms. Then, the carbon materials were used as adsorbents for methylene blue (MB) and methyl orange (MO) in aqueous solutions. KOH and urea-activated carbon (SCG-KU) revealed specific properties, including a large surface area (1665.45 m2 g−1) and excellent dye adsorption capacity. The influences of pH and temperature were investigated. The results of this work raise new possibilities for synthesizing carbon materials with high adsorption capacities from biowastes via less-toxic, energy-saving conventional pyrolysis methods for water-treatment applications. Next, we fabricated a cobalt oxide (CoO) nanoparticle-decorated nitrogen (N)-doped carbon catalyst (Co@SCG-KU) using SCG-KU with a simple method. SCG-KU was impregnated with cobalt chloride with urea and then pyrolyzed. Electrochemical performance results confirmed that the Co@SCG-KU had a high electrocatalytic effect (0.28 V at 10 mA cm-2) and better long-term stability. Finally, a uniformly dispersed ruthenium catalyst on a coffee waste-derived carbon support (Ru@SCC-KU) was successfully synthesized. Because SCG-derived carbon has a high surface area, rich porosity, and oxygen functional groups, Ru nanoparticles (NPs) could have provided uniform dispersion and suitable metal-support interaction. As-synthesized Ru@SCC-KU has excellent electrocatalytic activity and outperformed commercial catalyst platinum on carbon (Pt/C) for hydrogen evolution reaction (HER) in an alkaline medium involving 27.0 mV needed to reach a current density of 10 mA cm−2; 58.4 mV dec-1 of Tafel slope value. Moreover, the catalyst has excellent stability in alkaline media, showing negligible loss after the chronopotentiometry (CP) test for 24 hours. Our results demonstrate a simple, eco-friendly, low-cost method to prepare more efficient carbon materials for electrocatalyst and water purification from SCG.