Earth-abundant electrocatalysts for solar hydrogen production from water
Photoelectrochemical water splitting and photovoltaic electrolysis, are two processes that aim to use widely available resources like water and sunlight in providing hydrogen as a clean renewable energy for the world. The hydrogen evolution reaction (HER) is an integral electrochemical reaction in these solar to hydrogen (STH) technologies. In this dissertation, we aim to discover and understand HER catalysts. First, we take advantage of the rich chemistry of molybdenum sulfur compounds by using different precursors to tune the chemistry and structure of amorphous molybdenum sulfide catalysts (a-MoS [subscript x]). Particle catalysts were synthesized by either chemically oxidizing Mo₂S₁₂²⁻ or MoS₄²⁻ precursors to make MoS₆ and MoS₄ respectively, we found higher activity for the MoS₆ materials in this work showing the influence of sulfur stoichiometry on catalyst performance Next, in a bid to control the stoichiometry and structure of a-MoS [subscript x] materials even further, we used electrochemistry to synthesize thin films of these catalysts. By doing this, we were able to tune the stoichiometry, leading to the synthesis of a S/Mo ratios ranging viii from 2 to 6. Extended X-ray absorption fine structure (EXAFS) analysis, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy show that increasing S/Mo ratios of MoS [subscript x] catalysts leads to an increase in the density of shared sulfide groups which are the active sites for the HER on a-MoS [subscript x] materials. Finally, a known strategy to improve HER catalysis is using porous metal organic framework (MOF) precursors to make electrocatalysts. We synthesized Ni₂P in one step from single phosphine MOF precursors without the need for a second phosphorus source for the first time. Typical metal phosphide synthesis from MOFs usually require annealing separate MOF and phosphorus precursors. The MOF-derived nickel phosphide HER catalyst compared well to other catalysts reported in the literature and delivers current densities of 10 mA/cm² at an overpotential of 110 mV on glassy carbon substrate electrodes. Long term tests show this sample was stable for over 16 hours. Taken together, this dissertation focuses on making active catalysts either by increasing the density of active sites through chemistry or by making porous catalysts.