Versatile and tunable solvent-free synthesis of transition metal phosphides and their investigation as effective and stable hydrogen evolution electrocatalysts

Hydrogen-based energy has been identified as one of the promising alternatives to fossil fuels due to the merits of high energy produced per unit volume and zero emissions of carbon dioxide to the atmosphere. Current methods of hydrogen production are energy-intensive, expensive, and not environmentally friendly. Since water is an abundant hydrogen source, and a lot of attention has been given to splitting of water to produce constituent elements of hydrogen (H₂) and oxygen (O₂) in gaseous form using electrochemistry. Efficient generation of hydrogen from water splitting on a commercial scale is feasible via electrocatalyst materials. Earth-abundant non-precious metal-based electrocatalysts of low cost and high performance are highly desired versus expensive and rare platinum catalysts. In recent years, metal phosphides have emerged as one of the promising alternatives to platinum catalysts and are an intriguingly diverse class of compounds that show inherently useful physical and chemical properties for hydrogen evolution at the surfaces. Hydrogen generation activity of different metal phosphides is important to compare to identify effective and efficient catalysts, and similar preparations of metal phosphides are useful in better comparisons. In contrast to common synthesis methods that use toxic, non-ecofriendly reagents and high-cost processes, facile solvent-free tunable synthesis methods that can provide access a wide range of metal phosphide compositions are highly desired.

This dissertation details the rational, straight forward solvent-free preparation of crystalline, micro-meter sized particles of metal phosphide materials with different compositions (metal-rich to phosphorus-rich), complex mixed metal compositions (C_o1-xNi_xP₃) and advanced composites (metal phosphides on conducting carbon black and semi-conducting titanium dioxide) using simple modifications in reactions and use of non-toxic, low-cost reagents at moderate temperatures (500 °C). Crystalline metal phosphides show moderate hydrogen production on surfaces in aqueous acidic solutions and significant differences in hydrogen generation may be the influence of several physical and chemical differences of metal phosphide compositions. Advanced and complex metal phosphides such as mixed metal compositions show improved hydrogen generation between single metal counterparts. Developed synthesis methods are very effective and versatile towards preparations of both conducting and semi conducting composites of metal phosphides on inexpensive carbon black and titanium dioxide supports with high surface area. Generally similar generation of hydrogen is useful in composite materials with decreased amounts of expensive metal-based catalysts grown on inexpensive supports. All free-standing and supported metal phosphides show variable stabilities and survival after extended periods of electrocatalysis.

Facile, solvent-free synthesis routes that are very tunable and versatile towards a large range of metal phosphide compositions and advanced compounds are useful in materials synthesis. Hydrogen production on metal phosphides is complex and may influence by multiple factors but in general, phosphorus-rich surfaces are very effective and stable towards hydrogen generation. Further improvements of hydrogen generation on metal phosphide surface can be achieved via different metal mixing into a phosphide structure and uniform dispersions of less catalyst on to high surface area conducting supports.