How to store solar energy more cost-effectively for use at night

November 7, 2014

How electrolysis could produce hydrogen as a way to store renewable energy: During the day, solar panels supply surplus electricity for electrolysis, producing hydrogen from water, which is then stored in a tank. At night, hydrogen would be combined with oxygen from the air in a fuel cell to generate electricity. (Credit: Jakob Kibsgaard)

There’s currently no cost-effective, large-scale way to store solar energy, but Stanford researchers have developed a solution: using electrolysis to turn tanks of water and hydrogen into batteries. During the day, electricity from solar cells could be used to break apart water into hydrogen and oxygen. Recombining these gases would generate electricity for use at night.

There’s one major problem. Electrolysis uses electricity to crack the chemical bonds that hold H2O together. Cracking the chemical bonds of water produces a hydrogen ion — a proton with no electron to balance it out. A good H2 catalyst gives the proton a place to stick until it can pick up an electron to form a hydrogen atom on the catalyst surface and then pair up with a neighboring hydrogen atom to bubble off as H2.

The trick is finding a catalyst with the right stickiness. “If the binding is too weak, the ions don’t stick,” said chemical engineering Professor Thomas Jaramillo. “If it’s too strong, they never get released.”

Platinum is perfect, but pricey. Last year the Stanford engineers discovered that a version of molybdenum sulfide called molybdenum phosphosulfide, a catalyst widely used in petrochemical processing, had some of the right properties, with an efficiency approaching that of platinum, to serve as a cheap but efficient alternative to platinum, as described in the German scientific journal Angewandte Chemie.

Now members of Jaramillo’s group are working to improve this new catalyst. For instance they are engineering the material at nano-scale dimensions to catalyze the reaction more effectively.

The new catalyst could also help produce vast quantities of pure hydrogen through electrolysis. Today, pure hydrogen is a major commodity chemical that is generally derived from natural gas. Tens of millions of tons of hydrogen are produced each year; industrial hydrogen is important in petroleum refining and fertilizer production.

Abstract of Molybdenum Phosphosulfide: An Active, Acid-Stable, Earth-Abundant Catalyst for the Hydrogen Evolution Reaction

Introducing sulfur into the surface of molybdenum phosphide (MoP) produces a molybdenum phosphosulfide (MoP|S) catalyst with superb activity and stability for the hydrogen evolution reaction (HER) in acidic environments. The MoP|S catalyst reported herein exhibits one of the highest HER activities of any non-noble-metal electrocatalyst investigated in strong acid, while remaining perfectly stable in accelerated durability testing. Whereas mixed-metal alloy catalysts are well-known, MoP|S represents a more uncommon mixed-anion catalyst where synergistic effects between sulfur and phosphorus produce a high-surface-area electrode that is more active than those based on either the pure sulfide or the pure phosphide. The extraordinarily high activity and stability of this catalyst open up avenues to replace platinum in technologies relevant to renewable energies, such as proton exchange membrane (PEM) electrolyzers and solar photoelectrochemical (PEC) water-splitting cells.