To lower the costs of solar fuels and thermal energy storage, concentrated solar thermal (CST) researchers are working on increasing solar thermochemistry’s performance and redox cycle-ability.
Researchers at Khalifa University of Science and Technology have designed a perovskite mix that promises to achieve both goals. They described their results at the 30th SolarPACES Conference in the paper Two-Step Thermochemical Conversion of CO₂ using a Novel Nd1-xSrxMnO₃ Perovskite.
A novel perovskite designed to improve solar fuels yield
“This is a new class of perovskites that has never been studied for thermochemical applications, and our results show that this is a potential candidate for scalable renewable energy applications,” said co-author Khalid Al-Ali in a call from Abu Dhabi.
“Ceria is a standard material widely researched in thermochemical cycles, but it faces significant challenges. These include a low oxygen capacity due to ceria’s limited non-stoichiometric range (δ in CeO₂ -δ), and operation at excessively higher temperatures, leading to lower solar to fuel efficiencies. With perovskites, I think we can eliminate these problems by tailoring the composition of perovskite to achieve faster kinetics and higher yields.”
Perovskites can have various compositions, where mixing and doping specific components allow researchers to tailor their properties for specific applications.
“We need materials where we can modulate oxygen vacancy transport as we want easy transport of oxygen vacancies through the material,” he said.
“The current situation with ceria is that we have low yield due to ceria’s inherent thermodynamic limitations, and that’s why we are looking for better formulations in perovskite materials. This is the biggest obstacle or constraint that we are facing related to these materials. So that’s why we are looking for a new class. Perovskites are one class of oxygen exchange materials that can work at the very high temperatures we need for solar fuels.”
The researchers isolated a particular perovskite mix and tested it at lab bench scale recently, to understand the kinetics and correlate the experimental results with the theoretical results. Once that is confirmed, they will work on increasing the performance.
The paper describes that test:
“The reactivity of the air-calcined perovskite samples was studied using thermogravimetric analysis (TGA) with a SETARAM SETSYS apparatus. Approximately 100 mg of the sample was placed in a platinum crucible. The samples were subjected to the thermal reduction step in the TGA at 1400℃ with a heating ramp of 20 ℃/min under an inert argon gas flow of 0.020 NL/min, with 99.999% purity and less than 2 ppm O₂ .
The temperature was kept constant at 1400 ℃ for 45 minutes. Subsequently, in the oxidation step, the temperature was lowered to 1050 ℃ with a ramp of 20 ℃/min and the gas environment was switched to 50% CO₂ in Ar at a total flow of 0.020 NL/min for 60 minutes, completing the first cycle. During the second cycle, the temperature was raised from 1050 ℃ to 1400 ℃ under argon environment, and thermal reduction was carried out as before for 45 minutes. A similar re-oxidation step was carried out, lowering the temperature from 1400 ℃ to 1050 ℃ under CO2/Ar environment.”
“We found that the role of strontium as a side dopant in our material improves the overall rate of yields during these two-step cycles while maintaining structural stability,” Al-Ali said.
“The manganite perovskites is the most researchable family demonstrating significant fuel yield potential. Our study revealed that 40% strontium-doped neodymium-based manganite perovskites achieve the highest oxygen and carbon monoxide production yields. The material demonstrated potential for solar fuel production, achieving 103.7 micromoles of oxygen and 189.2 micromoles of carbon monoxide per gram.”
The advantages of perovskites for solar fuels
Perovskites can undergo redox cycling over a broad temperature range, typically from 500°C to 1000°C or higher. This wide operating range makes them suitable for high-temperature applications, like producing solar fuels.
Their research is applicable in the two-step thermochemical process to convert CO₂ into carbon monoxide (CO), a useful fuel and chemical feedstock. Researchers are working on the synthetic production of drop-in liquid fuels made from renewable carbon monoxide and hydrogen sources using concentrated solar power to replace fossil fuels.
“In the first step, at 1400°C, the material is thermally reduced using concentrated solar energy under inert environment,” Al-Ali explained.
“At this high temperature, the material releases oxygen, creating oxygen vacancies in its structure. This is the reduction phase of the redox cycle. In the second step, at a lower temperature, the reduced material reacts with water (H₂O) and carbon dioxide (CO₂). If oxygen is taken from water, we produce hydrogen (H₂); if oxygen is taken from carbon dioxide, we produce carbon monoxide (CO). Together, carbon monoxide and hydrogen form syngas. The cycle then repeats: in the first step, the material loses oxygen, and in the second step, oxygen is replenished.”
This research is primarily geared toward solar fuel production. However, due to the high temperatures possible with this perovskite mix, it could also be used for very efficient thermal energy storage, a crucial addition in most solar fuel production because this process generally must run continuously day and night.
Why reducing CO₂ to CO helps the climate
“We are emitting a lot of carbon dioxide in the atmosphere. So to find a way to utilize this carbon dioxide, we need to reduce it to carbon monoxide,” he noted.
“As carbon monoxide, it has a lot of industrial applications. We can produce fuel, methane or ethanol, and many chemical products. Carbon monoxide has several routes for chemical applications, such as producing solar based kerosene or liquid fuels. We are exploring these perovskites for CO₂ reduction to CO. The objective here is to produce pure carbon monoxide. We want to optimize the composition of the material that enhances this CO₂ reduction reaction.”
by Susan Kraemer
