Low-Dimensional FeS2 Nanostructures for use as Hydrogen Evolution Electrocatalysts

Abstract

The water splitting hydrogen evolution reaction which is used as an alternative method of hydrogen production currently relies on platinum group metal electrocatalysts to function. Currently this method accounts for only a small portion of actual hydrogen production. To achieve broad utilization of this method of hydrogen generation it would be beneficial if these precious metal catalysts were replaced with low-cost materials that use easily scalable production methods. Many different materials have been investigated attempting to identify effective replacements for precious metal electrocatalysts and FeS2 nanomaterials are a class of materials that meet these utilization requirements and have a demonstrated catalytic ability. This work examines the synthetic parameters and fundamental reaction mechanism involved in the formation of novel low dimensional iron disulfide nanostructures and subsequently details their development as replacement electrocatalysts. The low-dimensional hyperthin FeS2 nanostructures were generated by a scalable modified hot-injection technique. Maintaining low reaction temperatures kinetically confined the reaction to its early stages. Subsequent manipulation of the sulfur in the reaction allowed for the formation of novel one (1D) and two (2D) dimensional nanostructures which were observed in both transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Furthermore, increased dwell times allowed for stoichiometric control of products to produce a 1:2 Fe:S structure as characterized by energy dispersive X-ray (EDS), X-ray diffraction (XRD), and Raman spectroscopy. Additionally, these nanomaterials did not adhere to the normally observed reaction pathway and formed without undergoing growth through Ostwald ripening or orientated attachment. Electrocatalytic performance of 1D and 2D FeS2 materials coated on a glassy carbon electrode was tested against platinum as well as a common 3D FeS2 structure in a phosphate buffered solution (PBS) at neutral pH using linear sweep voltammetry (LSV). The near zero onset potential of the 2D structures was similar to platinum and both had calculated charge transfer coefficients of 0.71. Exchange current densities calculated using Butler-Volmer equations from best-fit lines yielding 2.2 and 8.0 μA cm-2 for the 2D structures and platinum, respectively. Scanning electrochemical microscopy (SECM) confirmed the formation of hydrogen and subsequent stability test showed stable performance for over 125 hours demonstrating that functional low-dimensional FeS2 nanomaterials are promising platinum electrocatalyst replacement.