The scientists of the University of the Basque Country in Spain have developed and tested a new method for producing hydrogen that operates at much lower temperatures than most available thermochemical technologies. The process is based on cobalt oxide (CoO) participating repeatedly in chemical reactions, acting as a kind of oxygen carrier. According to the research authors, their approach paves the way for using low-grade industrial heat and relatively inexpensive solar thermal systems for hydrogen production.
Hydrogen is considered one of the key energy sources of the future, but its production remains a challenging task. For example, direct thermal decomposition of water requires temperatures above 2,500 °C, and the most common industrial technologies are based on natural gas processing and accompanied by carbon dioxide emissions. Thermochemical cycles using special oxygen carriers are emerging as an alternative. But even these typically require heating above 1,300 °C, which significantly limits the range of available heat sources.
The Spanish researchers have proposed to use in the cycle a water gas reaction in which carbon monoxide (CO) acts as a reducing agent rather than pure water. In their two-stage process, cobalt oxide first reacts with CO at a temperature of about 350°C, converting to metallic cobalt and releasing pure carbon dioxide. The metallic cobalt is then oxidized by steam at 400°C, returning to its original state while simultaneously producing hydrogen. Both reactions occur at temperatures accessible to many low-temperature heat sources ranging from industrial waste heat to solar thermal plants.
To test feasibility of the idea, the scientists conducted a series of experiments using high-precision thermobalances, monitoring changes in the sample mass and hydrogen evolution. The very first experiments with pure CoO confirmed the principle: over two cycles, they managed to produce about 2.75 mmol of H₂ per gram of material. Moreover, the system demonstrated excellent reversibility as in the second cycle, slightly more hydrogen was released than in the first one. X-ray diffraction analysis confirmed that as expected, an intermediate metallic phase of cobalt is indeed formed during the process.
Then the researchers made an attempt to improve the material by adding other components. Replacing 10% of the cobalt atoms with nickel (NiO) had a noticeable effect: over two cycles, hydrogen yield increased by 33% compared to pure CoO. Nickel turned out to play a dual role: it facilitates cobalt oxide reduction, and, what is more important, acts as a physical barrier preventing the particles from sintering. The added iron oxide (FeO) also increased efficiency, but to a lesser extent.
Even more interesting results were obtained by adding titanium dioxide (TiO₂) nanoparticles at a concentration of 5% by weight. In just two cycles, hydrogen production increased by 56% compared to pure CoO. Examining the samples with an electron microscope, the scientists observed that the TiO₂ particles distributed around the cobalt, preventing its clumping during the repeated cycles. Essentially, they function as microscopic spacers, maintaining a high active surface area.
The best results came from combining both strategies – adding nickel oxide and titanium dioxide nanoparticles simultaneously. This material, designated TiNi-CoO, produced 24.81 mmol of hydrogen per gram of material over ten cycles. By comparison, pure cobalt oxide produced 12.72 mmol of hydrogen per gram over the same period. Moreover, the system’s efficiency gradually increased: the yield being 1.95 mmol per gram in the first cycle, reached 2.66 mmol per gram by the tenth cycle.
Why does performance improve from cycle to cycle? Analysis has shown that during cobalt oxide reduction, some of the carbon monoxide decomposes to form carbon deposits. This process is usually considered undesirable as it can lead to catalyst degradation. However, in this case, the carbon played a beneficial role. It accumulated between the cobalt particles, preventing their clumping together, and then, facilitated a gradual breaking down of large aggregates into the smaller ones. As a result, the active surface area of the material increased, and with it, the hydrogen yield grew.
Practical significance of this development is the ability to utilize heat sources often remaining underutilized today. The temperatures of 350–400 °C are available in many industrial processes, as well as in a number of solar thermal technologies. In addition, carbon dioxide is released in a concentrated form during the first stage, which simplifies its subsequent capture.
The technology still has a long way to go before it reaches industrial application. All the experiments to date have been conducted on a laboratory scale, with small amounts of material. Nevertheless, the research demonstrates that new combinations of materials can significantly expand the possibilities of thermochemical hydrogen production and make such processes more efficient and flexible.
