Redox Kinetics Study of Fuel Reduced Ceria for Chemical-Looping Water Splitting

Chemical-looping water splitting is a novel and promising technology for hydrogen production with CO2 separation. Its efficiency and performance depend critically on the reduction and oxidation (redox) properties of the oxygen carriers (OC). Ceria is recognized as one of the most promising OC candidates, because of its fast chemistry, high ionic diffusivity, and large oxygen storage capacity. The fundamental surface redox pathways, including the complex interactions of mobile ions and electrons between the bulk and the surface, along with the adsorbates and electrostatic fields, remain yet unresolved. This work presents a detailed redox kinetics study with emphasis on the surface ion-incorporation kinetics pathway, using time-resolved and systematic measurements in the temperature range 600–1000 °C. By using fine ceria nanopowder, we observe an order-of-magnitude higher hydrogen production rate compared to the state-of-the-art thermochemical or reactive chemical-looping water splitting studies. We show that the reduction is the rate-limiting step, and it determines the total amount of hydrogen produced in the following oxidation step. The redox kinetics is modeled using a two-step surface chemistry (an H2O adsorption/dissociation step and a charge-transfer step), coupled with the bulk-to-surface transport equilibrium. Kinetics and equilibrium parameters are extracted with excellent agreement with measurements. The model reveals that the surface defects are abundant during redox conditions, and charge transfer is the rate-determining step for H2 production. The results establish a baseline for developing new materials and provide guidance for the design and the practical application of water splitting technology (e.g., the design of OC characteristics, the choice of the operating temperatures, and periods for redox steps, etc.). The method, combining well-controlled experiment and detailed kinetics modeling, enables a new and thorough approach for examining the defect thermodynamics in the bulk and at the surface, as well as redox reaction kinetics for alternative materials for water splitting.