Characterization of a Laser-Based Heating System Coupled With In Operando Raman Spectroscopy for Studying Solar Thermochemical Redox Cycles

Abstract

A 200 W CO2 laser-based heating system coupled with in operando Raman spectroscopy has been developed. The system delivers highly concentrated radiation capable of driving thermochemical reactions and simulates heat fluxes expected by 3D solar concentrating systems. 10 mol% Gd-doped and pure ceria pellets were prepared and used to characterize the system because of their well-established thermodynamic and kinetic properties, as well as their strong Raman peak due to F2 g symmetrical mode at 460 cm−1. Reduction in an H2 atmosphere has been carried out to investigate the behavior of the full width at half maximum (FWHM) of the F2 g Raman peak resulting from changes in temperature and oxidation state. For both samples, an increase in temperature during heating in air (i.e., fully oxidized) resulted in a peak shift toward low wavenumber and an increase of FWHM. The FWHM versus temperature curves were then measured for controlled reduction extents ranging between sample averaged nonstoichiometries of δ = 0–0.209 as a function of temperature. At a fixed temperature, Gd-doped ceria exhibited an increase in FWHM with increasing reduction extent until δ = 0.056. At greater reduction extents, the FWHM decreased with increasing reduction extents. We attribute this to changes in the lattice parameter caused by the eventual formation of intermediate cubic Ce2O3 at the radiated surface. This study demonstrates the promise of utilizing Raman spectroscopy to probe thermochemical reactions in operando. Going forward, we expect that this will be an especially promising tool for characterizing emerging thermochemical materials with complex phase equilibria, especially for nonequilibrium processes.