(530b) Electrocatalysis Under Cover: Enhanced Hydrogen Evolution Reaction (HER) Via Defective Graphene Covered Pt(111)

The production of hydrogen through water electrolysis using renewable electricity is a promising carbon-neutral technology. We present insights into the hydrogen evolution reaction (HER) in aqueous H₂SO₄ (0.005 to 0.5 M) on Pt(111) and Graphene/Pt(111) under ambient temperatures and pressures. Graphene was grown on Pt(111) using chemical vapor deposition (CVD) of hydrocarbons [1]. Cyclic voltammograms (CV) and HER currents of Pt(111) in H₂SO₄ were quantitatively benchmarked to literature to ensure reproducibility and comparability of kinetic measurements [1-3]. CV sweeps (0.005 to 0.5 M, 23°C, 0.1 to 0.8 V_RHE) indicate that graphene overlayers are selectively permeable to H⁺ ions in the electrolyte, allowing H⁺ ions into the confined layer between Pt(111) and graphene while excluding SO₄^2- and other anions [2]. The extent of defects in graphene was quantified by integrating the anion adsorption region from CVs. The anion adsorption coverage on electrochemically pre-treated Graphene/Pt(111) (250 cycles between 0.1 to 1.2 V_RHE in 0.005 M H₂SO­₄) increases with HER reaction time at a rate of ~3*10^-6 mol_anion mol_Pt^-1 s^-1 while untreated Graphene/Pt(111) exhibits a slower increase of ~5*10^-7 mol_anion mol_Pt^-1 s^-1. Though the defect generation rate is slower on untreated Graphene/Pt(111), a sudden order-of-magnitude increase in anion coverage concurrent with transformation of the Pt(111) to polycrystalline Pt is observed after ~10 hours of HER. In contrast, no sudden increase in the anion capacity is observed on electrochemically pre-treated Graphene/Pt(111) for up to 15 hours of HER, though a transition from Pt(111) to polycrystalline Pt is also observed after ~10 hours of HER.

Initial HER currents on both pre-treated and un-treated Graphene/Pt(111) are slower than Pt(111) by 1 to 2 orders of magnitude (0.005 to 0.5 M, 23°C, -0.08 V_RHE). HER currents on both pre-treated and un-treated Graphene/Pt(111) increase with HER reaction time to values equal to or higher than that of Pt(111), concurrent with increasing graphene defects as probed using CV and SEM. Unchanged HER Tafel slopes and [H⁺] reaction orders suggest the same HER mechanism between Pt(111) and Graphene/Pt(111). We postulate that untreated graphene is mass transfer limited and HER currents transition from mass transfer limited to kinetics limited with increasing graphene defects. Li et al. computes that a graphene overlayer would weaken the hydrogen binding energy to Pt(111), shift the reaction closer to the peak of the HER volcano plot, and thus increase HER currents [6,7]. Though our results agree with prediction, we cannot negate other possible explanations for the observed increase in HER rate. These results demonstrate the promise of overlayer and confinement modifications in designing catalysts with properties closer to achieving optimum currents.

References

[1] Ferrari, A. C., et al., Physical Review Letters 97.18 (2006): 187401.

[2] Fu, Y., et al., Angewandte Chemie International Edition 56.42 (2017): 12883-12887.

[3] Marković, N. M., et al., Journal of Physical Chemistry B 101 (1997): 5405-5413.

[4] Gómez, R., et al., The Journal of Physical Chemistry 97.18 (1993): 4769-4776.

[5] Kita, H., et al., Journal of Electroanalytical Chemistry 334.1-2 (1992): 351-357.

[6] Li, H., et al., Proceedings of the National Academy of Sciences 114.23 (2017): 5930-5934.

[7] Greeley, J., et al., Nature Materials 5.11 (2006): 909-913.