(242d) Operational Limits of Redox Metal Oxides Performing Thermochemical Water Splitting

Abstract:

Solar thermochemical hydrogen production is an attractive technology that enables the storage of intermittent solar energy in the form of chemical bonds. Efficient operation requires the identification of a redox-active metal oxide (MO_x) material that can achieve high conversion of water to hydrogen at minimal energy input. The water splitting occurs by consecutive reduction and re-oxidation reactions. The MO_x is reduced to MO_x-δ and, in the second step, is re-oxidized by water to recover the initial MO_x. The material must reduce to MO_x-δ at temperatures achievable in concentrated solar receiver/reactors, while maintaining a thermodynamic driving force to split water. At equilibrium, extent of reduction depends on temperature and oxygen partial pressure, and in this analysis, a set of thermodynamic properties, namely enthalpy, and entropy of oxygen vacancy formation, is sufficient to represent the MO_x. The work presented here is a method to easily classify materials based on these thermodynamic properties under any condition of oxygen partial pressure and temperature.

Methodology:

This work extends the thermodynamic analysis performed by Bayon et al. ^[1] and Li et al. ^[2]. The chemical equilibrium of the thermal reduction reaction at T_red and pO_2,redis given by:

Δh_o,red - Δs_o,red T_red= -1/2RT_red ln((pO_2,red)/(p°)) (1)

And the equilibrium for the reoxidation reaction is analogous and described by the conversion of H₂O into H₂ (θ) as well as the equilibrium constant K_ws:

Δh_o,ox - Δs_o,ox T_ox= -RT_ox ln((1-θ)/θ K_WS) (2)

This work is based on the countercurrent reactor and the definition of the thermochemical properties at the outlet of the reduction and inlet of the oxidation provides the operational limits in terms of T_red, pO_2,red, T_ox and θ. Using Equations 1 and 2, the enthalpy of oxygen vacancy formation can be obtained from the entropy and temperatures of reduction and re-oxidation separately. To study the effect of these system variables, we hold fixed the reduction oxygen partial pressure at the inlet of the reduction reactor and the conversion of water into hydrogen leaving the re-oxidation reactor.

Advance of results:

An example of the application of the thermodynamic method, this section reports the system effects of for reduction oxygen partial pressure of 10^-5 and a value of conversion from H₂O to H₂ of 10%. By overlapping both equilibriums lines described in equation 1 and 2, we can obtain Figure 1. Generally, the higher the enthalpy of oxygen vacancy formation and the lower the entropy of oxygen vacancy formation,results in lower temperature differences between reduction and re-oxidation (note the difference of the height in the dashed and solid lines for 300, 400 and 500 kJ mol^-1). Only with high reduction temperatures is it possible to split water at high re-oxidation temperatures. Lowering the reduction temperature results in increasing the temperature difference between reduction and re-oxidation (see distance between both dashed and solid lines in Figure 1).

The intersection of both lines is the isothermal equilibrium temperature. Isothermal water splitting will only be possible, for a conversion of 10% of water, at a temperature of 2030 K and for unique pair of values of enthalpy and entropy. The pairs of and are: 300 kJ mol^-1 and 100 J mol^-1 K^-1; 400 kJ mol^-1 and 149 J mol^-1 K^-1; and 500 kJ mol^-1 and 200 J mol^-1 K^-1. We cannot find materials in the literature that can meet the isothermal criteria for oxygen partial pressure of 10^-5 in reduction and water conversion of 10% in re-oxidation. Either the values of entropy at a given enthalpy are too high or the values of enthalpy at a given entropy are too low. Above that intersection of oxygen partial pressures, we could potentially re-oxidize the material at a higher temperature than the reduction.

The work presented in this abstract will be extended to cover a wide range of conditions of oxygen partial pressure and temperatures. In addition, we will be presenting a classification of redox materials in operational ranges using the method briefly presented here. Despite the simplicity of the method, we believe this analysis will support future research in targeting thermodynamic properties of redox active metal oxides.

References:

[1] A. Bayon, A. de la Calle, K. K. Ghose, A. J. Page, R. McNaughton. Int. J. Hydrogen Energy 2020, 45, 12653–12679.

[2] S. Li, V. M. Wheeler, A. Kumar, M. B. Venkataraman, C. L. Muhich, Y. Hao, W. Lipiński. Energy Technol. 2021, 2000925 (19), 1–18, DOI: 10.1002/ente.202000925.