Preparation of nitrogen-doped multiwalled carbon nanotubes anchored 2D platinum dichalcogenides for application as hydrogen evolution reaction catalysts

Abstract

The alkaline hydrogen evolution reaction (HER) (H2O + 2e − → H2 + 2OH−) is fast gaining traction as a sustainable hydrogen gas generation route but suffers from slow reaction kinetics because of the additional water dissociation step and large reaction overpotential. As such, the current state-of-the-art acidic medium Pt and Ru catalysts suffer from considerable loss of catalytic activity in an alkaline medium. We propose the development and use of platinum metal dichalcogenides for alkaline HER. Platinum dichalcogenides are 2D materials that offer the advantage of more exposed catalytic sites, show dramatic chalcogen-dependent electronic properties, and have a band gap (0.24 eV - 1.8 eV for PtS2 and PtSe2) thus extending the use of these materials to light-stimulated photo-electrochemical (PEC) HER. As such, PtS2 is reported to be a semiconductor, PtSe2 is semi-conductive/semi-metallic depending on the number of layers, and PtTe2 is metallic. The Pt-chalcogen covalent bond intensifies down the chalcogen group. Additionally, the interlayer interactions in Pt dichalcogenides are covalent, and just like the Pt-chalcogen bond, intensify as the chalcogen atom changes from sulphur to selenium to tellurium. This behaviour of Pt dichalcogenides results from the Pt bonding d orbitals and the chalcogen bonding p orbitals that are relatively close in energy than in other TMDs, and the difference in the energy becomes smaller and smaller down the chalcogen group. Herein, we report on the synthesis of PtSe2 and PtTe2 using the colloidal synthesis method for the first time and then applying them as electrocatalysts in alkaline HER. As mentioned, developing 2D materials results in band gap development, particularly in PtS2 and PtSe2. Following this, PtSe2 was explored as a photocathode in light-induced photo-electrochemical HER. Generally, semiconductors are poor electron transporters and one of the major requirements for an efficient PEC cathode is solar absorption, charge generation, and efficient charge separation. The charge separation properties of PtSe2 were improved by supporting this material on highly conductive, mechanically, and thermally stable nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs). In Chapter 3, we report on the effect of varying selenium precursors from elemental selenium, sodium selenite to selenourea on the colloidal synthesis of PtSe2 in a mixture of oleylamine and oleic acid at 320 ℃. All the reactions resulted in the formation of PtSe2 although PtSe2 prepared from selenourea is amorphous, evidenced by relatively broader XRD peaks and a smaller crystallite size. HER activity of the three PtSe2 catalysts was evaluated in 1 M KOH at a scan rate of 5 mV/s and PtSe2 prepared from selenium exhibited the earliest onset potential of 46 mV, overpotential of 162 mV, and a smaller Tafel slope of 112 mVdec-1. This material exhibits the smallest resistance to electron transport and a high electrochemical surface area. We then explored the effect of altering tellurium precursor from elemental tellurium to tellurium tetrachloride, and sodium tellurite. Unlike the PtSe2 synthesis, different platinum tellurite phases, PtTe2, PtTe, and the mixed phase PtTe: PtTe2 were produced from Te, PtCl4, and sodium tellurite, respectively. Of the three, PtTe2 exhibited the highest alkaline HER activity with an onset potential of 29 mV, an overpotential of 107 mV, and a Tafel slope of 79 mVdec-1. In the same chapter, we compared the catalytic activity of PtSe2 (prepared from Se) and PtTe2 (prepared from Te) catalysts. We determined that PtTe2 has a high surface roughness and electrochemical surface, leading to relatively higher activity than PtSe2. However, PtTe2 is metallic and therefore does not have a band gap, which implies that it cannot be employed in light-stimulated catalysis reactions. In Chapter 4, we explored the use of PtSe2 as a light-stimulated PEC alkaline HER catalyst. We used in situ colloidal synthesis to grow PtSe2 on the walls of N-MWCNTs to improve the overall electron transport properties of PtSe2. PtSe2 anchored on N-MWCNTs was also studied in the dark and under illumination using 1 sun (100 mW/cm2) to determine the influence of light on the HER catalytic activity of the hybrid materials. This study demonstrates that the light-stimulated HER activity of PtSe2 improves when minimal amounts of N-MWCNTs are incorporated in the PtSe2 sample matrix. This then leads to employing these materials as photocathodes in PEC HER.