NaOH and Ni-assisted Pyrolysis of Coffee Grounds for the Co-production of Hydrogen and High-performance Carbon Nanosheets
A novel biomass conversion method of NaOH and Ni-assisted pyrolysis of coffee grounds for cogeneration of hydrogen and high-performance carbon nanosheet electrode materials was proposed.The effects of different conditions on the gas composition and gas yield were investigated.The structure of the carbon materials was characterized by SEM,TEM,EDS,XRD,Raman spectroscopy and N2 adsorption/desorption isotherms,and their electrochemical performance was tested.The results showed that when the mass ratio of NaOH to coffee grounds was 2,the amount of Ni nanoparticles added was 10％of the mass of coffee grounds,and the reaction temperature was 550 ℃,the highest hydrogen yield of 31.79 mmol/g was achieved,accounting for 84.46％of the gas products.Under this condition,the prepared carbon material was labeled 2-0.1-550.When the reaction temperature reached 600 ℃,the pyrolytic carbon sample 2-0.1-600 showed a porous carbon nanosheet structure with a specific surface area of 3 123 m2/g,a total pore volume of 1.808 cm3/g,and a micropore volume of 1.052 cm3/g,demonstarting a reasonable pore structure and appropriate degree of graphitization.The carbon material was used as an electrode material to obtain a high specific capacitance of 215 F/g(1 A/g)in the three-electrode system.The capacitive contribution rate of the fast and slow kinetic processes was further clarified by ultrafast charging/discharging kinetics.It was found by Dunn method that when the scan rate increased from 5 mV/s to 100 mV/s,the capacitance contribution from the fast reaction kinetics process increased from 64.34％to 96.50％.Furthermore,the symmetric electrode assembled in a 6 mol/L KOH solution electrolyte still achieved an energy density of 5.6 W·h/kg at a power density of 3 860.9 W/kg.Cyclic voltammetry(CV),galvanostatic charge-discharge(GCD),and electrochemical impedance spectroscopy(EIS)tests indicated that the material 2-0.1-600 exhibited ideal electrical double layer capacitor(EDLC)behavior and high ion transport efficiency,with a specific capacitance of 170 F/g at a current density of 1 A/g.Even when the current density increased to 10 A/g,a capacitor retention was still 94.1％(160 F/g).After 10 000 cycles at a current density of 5 A/g,the capacitance retention rate was 103％and the coulombic efficiency remained at 100％,indicating its excellent stability.
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