Dioscorea bulbifera seed peel chars as electrocatalysts for hydrogen evolution reactions—experimental and theoretical investigations

Abstract

Context

This study presents hydrochars derived from Dioscorea bulbifera seed peel as electrocatalysts for hydrogen evolution reaction (HER). These hydrochars are produced at 150 °C and 200 °C and respectively designated DBP@H_150 and DBP@H_200. FTIR, BET surface area measurement, and Boehm titration were used to characterize these hydrochars. Evaluation metrics such as double layer capacity (Cdl) gave information on how well they performed electrocatalytically for hydrogen evolution reaction. Interestingly, hydrochar made at 150 °C showed a Cdl values of 2.0, 1.0, and 0.2 mF at neutral, alkaline and acidic pH, while hydrochar made at 200 °C showed values of 1.5 and 1.2 and 1.1 mF at neutral, alkaline, and acidic pH. Electrochemical impedance spectroscopy (EIS) gave charge transfer resistance (R_ct) values of 48.0 and 60.0 Ω and linear sweep voltammetry (LSV) gave Tafel slope values of 96.2 and 124.1 mV dec⁻¹ for DBP@H_150 and DBP@H_200 respectively, showing the hydrochar’s exceptional electrocatalytic activities. The computed energy gap values from density functional theoretical (DFT) calculations for DBP@H_200 have the highest HOMO–LUMO gap of 5.688 eV, suggesting that it is more chemically stable. A very strong correlation of more than 0.8 is found to exist between energy gaps of hydrochars under acidic, alkaline, and neutral environments and their corresponding double layer capacitance (Cdl).

Method

Here, the structural and electronic properties of molecular systems are ascertained using a quantum chemical method known as density functional theory (DFT), and molecular properties are calculated using the density functional, B3LYP. Using the GAUSSIAN 09 program, DFT calculations were carried out at the B3LYP/6 − 31 g(d) level of theory. Molecular characteristics were calculated for Dioscorea bulbifera seed peel–derived hydrochars (DBP@H), including energy of the lowest unoccupied molecular orbitals (E_LUMO), energy of the highest occupied molecular orbitals (E_HOMO), and energy gap.