A thermochemical study of ceria: exploiting an old material for new modes of energy conversion and CO ₂ mitigation

Abstract

We present a comprehensive thermodynamic and kinetic analysis of the suitability of cerium oxide (ceria) for thermochemical fuel production. Both portions of the two-step cycle, (i) oxygen release from the oxide at 1773 and 1873 K under inert atmosphere, and (ii) hydrogen release upon hydrolysis at 1073 K, are examined theoretically as well as experimentally. We observe gravimetric fuel productivity that is in quantitative agreement with equilibrium, thermogravimetric studies of ceria. Despite the non-stoichiometric nature of the redox cycle, in which only a portion of the cerium atoms change their oxidation state, the fuel productivity of 8.5-11.8 ml of H 2 per gram of ceria is competitive with that of other solid-state thermochemical cycles currently under investigation. The fuel production rate, which is also highly attractive, at a rate of 4.6-6.2 ml of H 2 per minute per gram of ceria, is found to be limited by a surface-reaction step rather than by ambipolar bulk diffusion of oxygen through the solid ceria. An evaluation of the thermodynamic efficiency of the ceria-based thermochemical cycle suggests that, even in the absence of heat recovery, solar-to-fuel conversion efficiencies of 16 to 19 per cent can be achieved, assuming a suitable method for obtaining an inert atmosphere for the oxygen release step.

High-temperature isothermal chemical cycling for solar-driven fuel production

“…6,7 The time required for ramping up and down between the two temperatures is a variable depending on the type of reactor utilized. In a tube reactor heated by an infrared image furnace, 6 the rate can be as high as 1000 1C min À1 , whereas in solar reactor average rates of about 50 1C min À1 have been achieved. 7,11 These time scales, in combination with the thermodynamic prediction of 8.6 ml g À1 of hydrogen production, imply an overall rate of 7.6-12.5 ml g À1 h À1 .…”

“…2, Q solar consists of (1) the heat input to reduce ceria at the operational temperature and (2) that required to vaporize water and heat steam to the reaction temperature, modulated by the solar absorption efficiency. For notational convenience and consistency with our previous work 6 we consider the ceria non-stoichiometry at the initiation of the oxidation half-cycle to be the initial value (d i ). For a change in ceria nonstoichiometry of Dd = d i À d f upon reduction (where d f is the final value), the first term is…”

“…[1][2][3][4][5][6][7] The reaction scheme, as written out explicitly for the case of H 2 production from H 2 …”

Splitting CO2 with a ceria‐based redox cycle in a solar‐driven thermogravimetric analyzer

“…Due to its favorable oxidation thermodynamics, rapid reaction kinetics, and morphological stability over a wide range of temperatures, ceria (CeO 2 ) is currently considered the benchmark redox material 2, 3, 4, 5, 6, 7. The two‐step thermochemical cycle is then represented by:…”

Modelling of solar thermochemical reaction systems

“…Ceria (cerium dioxide) has been identified as a candidate redox material to split H 2 O and CO 2 in solar thermochemical fuel production cycles (Chueh and Haile, 2010). Ceria reduces nonstoichiometrically without phase change, which is beneficial in terms of material stability, and also remains solid throughout the two-step cycle, which simplifies the separation of the gaseous products from the redox intermediary material.…”