The environmental impact of finite fossil energy resources has spurred ever increasing research into renewable energy resources (e.g., solar and wind electricity and solar fuels). These intermittent energy sources entail a need for storage and conversion. Energy can be stored safely and conveniently as chemical bonds, and hydrogen (H2) is one such possible energy carrier. H2 is also a valuable bulk chemical currently produced at large-scale (˜63 Mton/yr3) from natural gas. A renewable source of H2 would thus not only be useful for storing intermittent renewable energy, but also serve as a major chemical feedstock.
Although H2 can be produced by electrochemical water splitting, industrial electrolyzers, even where inexpensive electricity is available, operate with high electrical losses due to inefficient catalysts (50-65%) or have high capital costs. Commercial alkaline electrolyzers rely on Ni electrodes that have low efficiencies and degrade over time, but this limitation has been accepted due to the relatively low cost of Ni. Polymer electrolyte membrane (PEM) type electrolyzers use acidic electrolytes and Pt electrodes, the archetypical H2 evolution catalyst owing to its high efficiency for the H2 evolution reaction (HER). On Pt(110) there is no intrinsic overpotential, and each 29 mV of applied potential generates a ten-fold increase in current (Tafel slope of 29 mV/dec). However, Pt is a scarce element (106 times more scarce than Fe) and very expensive. Consequently, research has focused on developing alternative less expensive, and robust catalysts made from earth abundant elements that are as efficient as Pt.
Molybdenum sulfides are excellent HER catalysts that have low overpotentials (0.1-0.15 V) and low Tafel slopes approximately 40 mV/dec. In addition, a Ni—Mo—N alloy on carbon has also been reported as an efficient HER catalyst with an onset overpotential of 78 mV and Tafel slope of 36 mV/dec. At alkaline pH, Ni or NiMo alloys are highly efficient (−20 mA/cm2 at −80 mV vs RHE) but both degrade in acidic media. Overcoming this instability would offer an inexpensive HER catalyst from abundant materials.
Molecular Ni complexes chelated by organophosphme ligands are active HER catalysts, with turnover frequency (“TOF”) of up to 106,000 s−1. The [001] facet of the solid state compound Ni2P was recently suggested as highly active for the HER by computational modeling (DFT). Popezun, E. J. et al., J. Am, Chem. Soc. (2013). There is a continued need, however, to develop alternative, cost-efficient, and efficient HER catalysts. The present invention addresses these needs among others.