A newly synthesized bipyridine-containing manganese( II ) complex immobilized on graphene oxide as active electrocatalyst for hydrogen gas production from alkaline solutions: Experimental and theoretical studies
Résumé
A new bipyridine-containing manganese (II) complex, namely [(BPy)2MnCl2]·0.5H2O (MnC), was synthesized and characterized. Spectral and X-ray structural features revealed that MnC has an octahedral environment around Mn(II). Graphene oxide (GO) was noncovalently functionalized with the MnC. MnC- and (MnC/GO)-modified glassy carbon (GC) interfaces with different loading densities (0.2, 0.4, 0.8, and 1.0 mg cm−2) were tested as electrocatalysts for efficient generation of H2 in 0.1 M KOH solution. The GC-MnC catalysts exhibited high electrocatalytic activity for the HER, which enhances upon increasing the catalyst's loading density. The GC-MnC catalyst's HER activity, at any tested loading density, was found to further increase upon supporting MnC on GO. The best performing electrocatalyst, namely GC-(MnC/GO) with a loading density of 1.0 mg cm−2, exhibited high HER catalytic activity with a low onset potential value of −22 mV vs. reference hydrogen electrode (RHE) and a high exchange current density of 0.75 mA cm−2. It delivered a current density of 10 mA cm−2 at an overpotential value of 119 mV. In addition, it recorded a Tafel slope value of −114 mV dec−1. These HER electrochemical kinetic parameters approached those measured here for the commercial Pt/C under the same operating conditions (−10 mV vs. RHE, 0.88 mA cm−2, 113 mV dec−1, and 110 mV to yield a current density of 10 mA cm−2) and also comparable to the most active molecular electrocatalysts for H2 generation from aqueous alkaline electrolytes published in the literature. The long-term stability of the best performing electrocatalyst, namely GC-MnC (1.0 mg cm−2)/GO, was evaluated using cyclic voltammetry (repetitive cycling up to 5000 cycles) and chronoamperometry (48 hours) techniques, and the obtained results revealed its promising stability. A plausible mechanism for the interaction of MnC with GO was proposed based on density functional theory (DFT) calculations. DFT calculations confirmed the catalytic role of the supporting material (GO) on the fabricated metal complex in catalyzing the HER.