Their work indicates that a synergistic effect of reduced LSF and metallic iron phases is attributable to the exceptional steam conversion. To further enhance this effect, they proposed a layered reverse-flow reactor concept. Using this concept, they achieved more than 77% steam to hydrogen conversion is achieved at 930 °C—15% higher than the maximum conversion predicted by the second law for unpromoted iron (oxides).

He and Li and other colleagues earlier last year had proposed a hybrid solar-redox scheme for the co-production of hydrogen and liquid fuels. The hybrid solar-redox process converts solar energy and methane into separate streams of liquid fuels and hydrogen through the assistance of an oxygen carrier. The new catalyst material improves the efficiency of that process, depicted in the schematic below.

Schematic of the hybrid process for liquid fuel and hydrogen generation. Image credit: Feng He. Click to enlarge.

We’re excited about the new material and process because it converts water, inexpensive natural gas and clean, renewable solar energy into valuable syngas and hydrogen fuels. We’re optimistic that commercial utilization of this technique could promote the efficient usage of solar energy and domestic natural gas, produce relatively low carbon dioxide emissions while making liquid transportation fuel, and generate low cost, high purity hydrogen.

—Feng He, lead author

Researchers have long known that iron oxide can be used as a catalyst for thermal water splitting, but it is not very efficient. The addition of LSF significantly improves iron oxide’s activity, making it far more efficient. The 77% conversion of the water used (in the form of steam) into hydrogen far outpaces the prior best conversion mark for thermal water-splitting of around 20%.

In the hybrid solar-redox process, methane is injected into a reactor that is heated with solar energy. That chamber contains the catalytic composite, which reacts with the methane to produce syngas and carbon dioxide. This process “reduces” the composite particles, stripping them of oxygen. The syngas is removed from the system and the reduced composite particles are diverted into a second reactor.

High-temperature steam is then pumped into the second reactor, where it reacts with the reduced composite particles to produce hydrogen gas that is at least 97% pure. This process also reoxygenates the composite particles, which can then be re-used with the methane, starting the cycle all over again.

Initially, the steam has to be produced with an external energy source, but once the cycle is initiated the chemical reactions produce enough heat to convert water into steam without an external heat source.

We’ve created the catalytic particles and conducted every step of this process, but only in separate batches. We’re now in the process of building a circulating bed reactor to operate this entire cycle in a continuous mode in real world conditions. Next steps include fine-tuning the catalytic compound to make it better and cheaper, improving the overall process, and developing better reactors.

—Feng He

The work was supported by the National Science Foundation under grant number CBET-1254351 and the US Army Research Office under grant number 61607-CH-RIP.

Resources

• Feng He and Fanxing Li (2014) “Perovskite promoted iron oxide for hybrid water-splitting and syngas generation with exceptional conversion” Energy Environ. Sci. doi: 10.1039/C4EE03431G

• Feng He, James Trainham, Gregory Parsons, John S. Newman and Fanxing Li (2014) “A hybrid solar-redox scheme for liquid fuel and hydrogen coproduction” Energy Environ. Sci., 7, 2033-2042 doi: 10.1039/C4EE00038B