Water splitting by a two-step temperature-swing reaction (left) and by isothermal reaction. (Credit: Roeb, Sattler; Science.)
What fascinates me about alternative energy is the diversity of the research portfolio. Some researchers study energy conversion systems (fuel cells, windmills, solar cells), some study energy storage systems (batteries, supercapacitors, flywheels), and some study fuels, whether alternatives to use in place of gasoline or fuels like hydrogen to power completely new systems. This story is about the last group.
A central attraction of the so-called hydrogen economy is its cleanliness. Hydrogen “burns” with oxygen to create benign water, and it does so very efficiently. But it cannot be mined or pumped. It needs to be peeled out of something—usually water.
For its part, hydrogen really likes being water. One approach to water splitting is to set up reactions that encourage the oxygen to leave the water molecule and liberate the hydrogen in the process. The idea is to use solar energy to provide the heat required to drive the reaction.
A long-known means to this end is the “two-step temperature-swing” water splitting reaction (shown on the left in the diagram). Step one is a metal oxide reduction reaction that releases oxygen. (The released oxygen is not important, it is the oxygen-poor metal oxide that matters.) Step two reacts the oxygen-deficient metal oxide with water. The thermodynamic driving force is such that the oxygen dumps the hydrogen for a more stable home in the metal oxide.
Some metal oxides used for water splitting are CeO2 and solid solutions of ferrites. Reducing these compounds takes a lot of energy, and, in fact, the reduction reaction proceeds in the 1,200–1,500˚C range.
It also takes a lot of energy to trigger the oxygen–hydrogen divorce, but not quite as much; thus the “temperature-swing” label. As diagram (a) shows, the water splitting oxidation reaction purrs along at about 1,000˚C, but at the cost of releasing a fair amount of heat to drop the reaction temperature to its sweet spot.
A recent paper in Science (subscription required for full text) notes that the two-step reaction process has the advantage of keeping the oxygen-generating and hydrogen-generating reactions separate from each other so they do not recombine to water. However, throwing away heat to effect the thermal cycling is thermodynamically inefficient, and induces thermal stresses in the solar-powered reaction vessel. (See more about advances in solar reactors in this previous CTT post.)
The authors from the University of Colorado (Boulder) challenged the conventional thermal cycling approach and looked for isothermal redox reactions. The UC group studied a reaction called the “hercynite cycle,” which is a redox reaction for decomposing iron oxide compounds by reacting them with other metal oxides, such as alumina. In a second step, the reaction products recombine in the presence of water to form the starting compounds again, plus hydrogen. The first reaction begins when temperatures reach 940˚C—a significantly lower temperature—and the reason the group used it.
The investigators studied this hercynite cycle based on cobalt ferrite at 1,350˚C:
CoFe2O4 + 3Al2O3 + heat → CoAl2O4 + 2FeAl2O4 + ½O2
CoAl2O4 + 2FeAl2O4 + H2O → CoFe2O4 + 3Al2O3 + H2.
According to the paper, the higher temperature leads to a larger fraction of Fe2+ ions, which determines the hydrogen-generating capacity of the material. The maximum possible is one H2 molecule for every two Fe2+ cations. Kinetics may have played a role, too. However, their results show that the reaction can be optimized by controlling the partial pressures. Referring to the second reaction, the article says, “by increasing the partial pressure of the oxidizing gas, we can drive the water-splitting reaction further toward the products. Indeed, an increased partial pressure of water resulted in higher H2 production capacities at higher rates.” The oxidizing gas here is steam.
The researchers also found that higher steam pressures raised the water chemical potential on the surface of the active material, which had the dual effect of increasing the thermodynamic driving force for the reaction and increasing the reaction rate simply by having more reactant available.
After proving that an isothermal hercynite cycle will produce hydrogen, the next question is, how much? According to the abstract: “… at 1350°C using the ‘hercynite cycle’ exhibits H2 production capacity >3 and >12 times that of hercynite [iron oxide] and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.”
In a “Perspective” commentary in the same issue of Science, German scientists note that the primary economic driver for solar processes (as this one would be) is the cost of the solar collectors. They offer a useful benchmark: A hydrogen generation system based on metal oxide decomposition, such as the hercynite cycle, would have to be at least 20% efficient to be a viable alternative to solar-powered low-temperature electrolysis. The commentators agree—this work seems to demonstrate a pathway for more hydrogen production, at lower temperatures, and more efficiently.
The paper is “Efficient Generation of H2 by Splitting Water with an Isothermal Redox Cycle,” by Christopher L. Muhich, Brian W. Evanko, Kayla C. Weston, Paul Lichty, Xinhua Liang, Janna Martinek, Charles B. Musgrave, and Alan W. Weimer; Science; 2 August 2013 (DOI: 10.1126/science.1239454).