Hydrogen, the fuel used in rocket engines, has long been the Holy Grail of zero-emissions energy research. Like the flying car, its promise is heralded by each generation as being right around the corner.
Hydrogen is the most abundant element in the universe, and it’s highly reactive. Because of this, it is scarcely found in its pure form on Earth. When paired with oxygen, the reaction produces water and a fair amount of photonic/thermal energy. This potential, or “specific energy” is 3.5 times higher than that of gasoline. As seawater, it is more abundant than petroleum crude.
The roadblock as a fuel of the future has always been the high amount of energy required to split water’s hydrogen-oxygen bonds. Water splitting can be accomplished through a variety of methods. Large-scale industrial production usually involves natural gas, i.e., methane, and a steam reforming process. In the presence of a nickel-based catalyst at high temperatures, methane and steam react to produce carbon monoxide and hydrogen.
Even factoring in the cost of compressing the gas into a transportable liquid, natural gas prices have (surprisingly) fallen precipitously as a result of fracking, so water-splitting now has production costs comparable to gasoline.
Electrolysis, passing an electric current through a liquid medium to separate its core components, has been used for 200 years to reduce oxygen and hydrogen to their gaseous states. For most of this process’ history, the high cost of producing the electricity needed to drive the reaction has outweighed the amount of energy derived from the hydrogen produced. Recent technology and manufacturing advances in clean energy generation, such as wind and solar, have made electrolysis more attractive.
Photovoltaic-powered electrolyzing systems, such as the one reported in the October 31, 2016, edition of Nature Communications, have achieved solar-to-hydrogen efficiencies (STH) of 30 percent.
STH measures the specific energy contained in the amount of hydrogen produced, versus the solar power required to drive the water-splitting process. The team coupled a three-junction solar cell array with a series of bi-polymer electrolytic membranes. The array was outfitted with solar tracking capabilities to maintain a consistent supply of power during daylight hours. The previous record for a setup of this type was 24 percent STH.
Water-splitting occurs in plants during photosynthesis, when photons from the sun kick off a series of chemical reactions that power “oxygen evolving complexes.” These complexes utilize a catalyst in the chloroplast to liberate hydrogen for use in creating the molecular fuels (e.g., ATP) that drive cellular machinery. Even though photosynthesis in nature has had a few hundred million years to evolve, it still only achieves an STH of six percent.
Researchers at the Department of Energy’s National Renewable Energy Laboratory reported in the March 13, 2017, edition of Nature Energy that their multi-junction semiconductor photoelectrochemical cells (PEC) had achieved an STH of 16 percent.
PECs, or artificial leaves, are “printed” in the same manner that solar cells and integrated circuits are produced. Rather than convert sunlight into electricity to be used by a secondary electrolyzer, water is passed over the surface of a cell in the presence of sunlight, which directly powers the microelectrodes and the electrolysis process.
This is accomplished through a special “doping” of the gallium-indium-arsenide substrate. Transistor-sized electrodes are laid down first, followed by a metamorphic outer covering that enables tuning each junction, thus reducing voltage loss and improving efficiency.
One approach to large-scale solar-to-hydrogen production comes from a team of researchers at the University of Tokyo. In the February 1, 2018, edition of Joule, they describe a multi-component system that makes use of a titanium oxide photocatalyst to separate the two elements.
Their design, simply put, is a one-square-meter sheet treated with a specialized flux, with a strontium titanium trioxide aluminum powder screen printed onto the surface.
Once the sheet is framed and covered with glass, water is passed over the treated panel as it is exposed to direct sunlight. The panel is tilted at a slight angle to promote gas transport. Bubbles move towards ceramic membranes that separate and collect the two gases.
Though the Tokyo team has not mapped out a method in which to store the hydrogen gas generated, they may not need to look any further than metal hydride storage systems already in use by hydrogen fuel cell vehicles.
Because of its peculiar properties and high reactivity to oxygen, pure hydrogen compression and storage is unsafe, as well as energy- and volume-intensive. Even as a liquid, hydrogen takes up space. But when paired with a hydride-forming metal such as palladium, zinc or titanium, a much higher concentration can be stored in the same volume.
Fuel cells generate electricity through the controlled oxidation of hydrogen via a solid electrolytic polymer. Incoming hydrogen reacts with a catalyst on the anode, which strips off its electron, leaving a positively charged proton. The proton is then free to pass through a non-conducting membrane towards the cathode. Once through, the proton will bind with oxygen, forming water via the circuit created by connecting the terminals. It is this circuit that provides us with electrical power.
A singularity of sorts is forming within hydrogen fuel technologies, as fuel cells already offer higher energy densities than lithium-ion batteries by a three-to-one margin. As solar-to-hydrogen technologies continue to improve, one can imagine a day when Exxon Mobil owns vast swaths of desert outfitted with arrays of PEC/PC panels, all slowly charging metal hydride tanks.