Could a Perovskite make Cheaper Solar Hydrogen?

A perovskite may be cheaper than ceria for solar-powered thermochemical hydrogen production, according to a new paper. A perovskite formulation that operates at a slightly lower temperature than ceria is being investigated by Xin Qian in the US Department of Energy-funded study, “Outstanding Properties and Performance of CaTi0.5Mn0.5O3-d for Solar-Powered Thermochemical Hydrogen Production”.

Solar thermochemistry works in two steps to produce solar hydrogen. Both steps require high temperatures of up to 1500 ° C, which are supplied in a solar reactor that is heated by rays of highly concentrated sunlight that are reflected by heliostats (mirrors) into a receiver. In the reactor, first a gas and then steam are forced through a porous material at high temperatures in order to split off hydrogen.

In the first step; Oxidation Reduction (Redox) An inert gas such as argon is forced through a porous monolithic redox material – usually a metal oxide – so that it undergoes thermal reduction to release its oxygen. At the second step; When the hydrogen is split off from water, steam is forced through the porous material. The oxygen from the water oxidizes the reduced metal oxide again, so that the hydrogen is now released.

Candidates for these porous redox materials are of increasing research interest. First, iron oxides were tried. However, these had severe sintering problems during the repeated cycles required for continuous hydrogen production. Cerium oxide (CeO2) is currently state-of-the-art and, due to its stable behavior at high temperatures, is already being commercialized for the production of solar fuels such as jet fuel or hydrogen.

“This kind of behavior is very interesting because when you reduce ceria it gradually releases oxygen, and when you oxidize it with steam it gradually fills in – it draws oxygen from the steam, filling in the oxygen vacancies in the structure it becomes oxidized again, ”said Qian, materials scientist at Northwestern University.

“Cerium oxide will therefore have very fast kinetics and very stable structures in many cycles. The main problem, however, is that the enthalpy of reduction is too high. So if you cut it down, you actually need temperatures above 1500 ° C to produce a lot of fuel. This high temperature causes very serious problems in the construction of the solar reactor. There will be many material problems. “

While some research and development in solar thermal chemistry is developing new materials for building high temperature solar reactors that can withstand such high temperatures, other solar researchers are looking for alternative redox materials like perovskites, which can behave like ceria, but at lower temperatures solar hydrogen can make hydrogen economical .

“Both materials have their advantages and disadvantages,” noted Qian. “In addition to the high temperature requirements and low fuel productivity of cerium oxide, another disadvantage is that cerium is a rare earth metal. Therefore, its cost is much higher than that of perovskites, which are very cheap with their earth-rich elements.”

“The advantage of cerium oxide, however, is that the material is very stable under extreme conditions up to very high temperatures. The kinetics of this perovskite is not as fast as that of ceria. However, it still has very fast kinetics for oxygen release, which means you can cycle it with a very short cycle time to produce around three milliliters of hydrogen per gram of oxide within 30 minutes, for example. So in terms of hydrogen productivity. We have achieved a very short cycle time. It’s a breakthrough. It’s higher. It is probably the highest of all the results reported. “

The Ti-doped calcium manganite (CaTi0.5Mn0.5O3-δ) evaluated with perovskite Qian is not stoichiometric: its atoms are not combined in exact integer ratios. It is created by doping the perovskite-calcium-manganite (CaMnO3-δ) with Ti.

He believes this perovskite could serve as an alternative to cerium oxide, which is used in solar thermal chemical hydrogen production. Laboratory tests showed a high hydrogen yield of 10 milliliters of hydrogen per gram of oxide in a thermochemical cycle in which the first step was at 1350 ° C and the second reoxidation step – to split off the hydrogen – at 1150 ° C.

But the search continues.

“Our material is good, but I don’t think it’s the end of the material search,” he noted. “We need to research further to push the limits of material performance. We need to discover a material with medium enthalpy and sufficiently high entropy to be thermodynamically favorable for both the thermal reduction and the water splitting steps. Given the very large compositional space of the perovskites, people can explore novel compositions, so we can expect a multitude of material discoveries. “

Like most research, the materials scientists on this team worked with a computer group to expedite the discovery process.

“With computational screening, you can actually remove a lot of materials that are not interesting. In fact, we don’t know which material has these properties. So if we don’t all have to measure, we’ll save a lot of time, “Qian said.

“For example, when I know that a material should have an enthalpy of reduction in the range between 200 and 300 – because for these types of materials we know that the redox thermodynamics of the material actually determine the capacity of fuel production – if so, calculate If you list all materials with the enthalpy of response in this range, then we don’t have to measure them all. “

But for this perovskite that has quantified its properties; its stability, crystal structure, phase transition, reduction dynamics and fuel production rate; The next step will be to see how quickly they can accelerate the rate of hydrogen production.

“Perhaps our next step is to see how you can tweak these properties for better material performance,” he commented. “For example, we would like to know whether this is limited by the diffusion of oxygen or the surface reaction of oxygen on the surface. In the case of highly porous sample architectures, the reaction is likely to be limited by the surface reaction step. A catalytic converter can then possibly be applied to improve the reduction rate and produce more fuel within a certain cycle time. “

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