Splitting of carbon dioxide on Ba2_2Ca0.66_{0.66}Nb1.34−x_{1.34-x}Fex_xO6_6 for clean fuel production

Limiting warming to 1.5 °C, in line with the Paris Climate Agreement, requires rapid emissions reductions across all sectors of the economy by 2050. However, there are sectors, often called the hard-to-decarbonise sectors, for which electrification and efficiency improvements alone will be insufficient. The production of carbon-neutral drop-in fuels could help to decarbonise aviation, shipping, and freight, while a carbon-neutral chemical precursor could reduce the chemical industry’s reliance on crude oil. This precursor could be carbon monoxide, which is already widely used in the chemical sector. Kerosene, diesel, and petrol can be synthesised from carbon monoxide and hydrogen via the Fischer Tropsch process. A thermochemical cycle based on Ba$_2$Ca$_{0.22}$Nb$_{1.34-x}$Fe$_x$O$_6$ (BCNF), a double perovskite, can convert carbon dioxide into carbon monoxide. BCNF perovskites with varying iron content ($x$ = 0, 0.34, 0.66 and 1) were synthesised and characterised before being thermally cycled between reduction under nitrogen and oxidation under carbon dioxide. The presence of iron was found to be crucial to the formation of oxygen vacancies and the ability to split carbon dioxide, where Ba$_2$Ca$_{0.22}$Nb$_{0.34}$Fe$_x$O$_6$ (BCNF1) was the only compound capable of splitting carbon dioxide. Optimising the reaction conditions for BCNF1 increased the CO production from 160 μmol/g to 447 μmol/g. Reacting over the perovskite as a powder increased yields to around 730 μmol/g. Increasing the mass of powder used from 1g to 18g, increased the CO yield further to 859 μmol/g. This makes BCNF1 one of the most successful materials in the literature while reacting at lower temperatures than other thermochemical materials. This is partly due to the low activation energy of the CO$_2$ splitting reaction of 46.62 kJ/mol, which is one of the lowest recorded in the literature. Additionally, BCNF1 powder has been proven to be thermally stable over 10 cycles, without experiencing the sintering and grain growth seen in BCNF1 pellets. Next, BCNF1 was scaled up to 100g to understand the redox activity at a larger scale. BCNF1 was found to split 10.1 % of CO$_2$ at 800 °C, with 100 % selectivity to CO$_2$. This conversion yield was used to perform a techno-economic analysis for an industrial CO production facility based on BCNF1. Such a facility was found to produce CO at well below the market price. A 150 m$^3$/hr facility can be profitable with electricity prices up to $0.39/kWh, allowing for short-term fluctuations in electricity prices without affecting profitability. At an electricity price of$0.15/kWh, CO can be produced at $0.26/kg, which is significantly cheaper than other carbon-neutral methods of carbon monoxide production including electrolysis. Finally, the sector coupling between the iron and steel sector and a thermochemical CO production facility is presented. The TC-BF-BOF system allows for the production of low emission steel through a closed carbon loop. If this system was applied to the BF-BOF facilities in the UK, steel sector emissions could be reduced by 88 %. Additionally, with an initial investment of £720 million, UK wide emissions could be lowered by 2.9 %. This investment would be completely repaid in 22 months due to significant savings from the replacement of expensive metallurgical coal. Finally, recommendations for future research are made

Thermochemical splitting of carbon dioxide by lanthanum manganites - understanding the mechanistic effects of doping

This review investigates the effect of different dopants on the oxygen evolution and carbon dioxide splitting abilities of the lanthanum manganites. Particular focus was placed on the lanthanide, alkaline earth metals, redox-active transition metal, and non-redox active Group 3 metals. The review suggests that a small ionic radius lanthanide on the A-site can increase the size discrepancy, leading to Mn-O6 octahedra tilting and more facile Mn-O bond breaking. Doping the A-site with a divalent alkaline earth element can increase the valance of the transition metal, leading to greater reduction capabilities. A transition metal with one electron in the eg orbital is the most effective for reduction while for oxidation, zero electrons in the high-energy eg orbitals is optimal. Finally, doping of the B-site with metals such as gallium or aluminium aids in sintering resistance and allows reactivity to remain constant over multiple cycles. Higher reduction temperatures and moderate re-oxidation temperatures also promote higher fuel yields as does the active reduction of the perovskite under hydrogen, although the total energy consumption implications of this are unknown. Far more is known about the mechanism of the reduction reaction than the oxidation reaction, therefore more research in this area is required