Invited Talk: The Role of Entropy in the Success of Nonstoichiometric Oxides for Solar Thermochemical Water and CO2 Splitting

  • Type:
    Conference Presentation
  • Conference Type:
    AIChE Annual Meeting
  • Presentation Date:
    November 16, 2020
  • Duration:
    15 minutes
  • Skill Level:
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Owing to their high theoretical efficiency, solar thermochemical water and CO2 splitting cycles provide a promising pathway to store solar energy in a stable chemical bond. Because direct thermolysis of H2O and CO2 proceeds at prohibitively high temperatures, the two-step cycle using a metal oxide intermediate has been extensively investigated as a practical lower-temperature process. In Step 1 of the redox cycle, a metal oxide MOox is reduced to MOred at high temperature Tred using concentrated solar energy to drive the reaction endotherm. In Step 2, the reduced oxide is cooled to Tox and H2O and/or CO2 are introduced. The now oxygen-hungry MOred is capable of stripping an oxygen from H2O or CO2 to produce H2 or CO fuel, while recovering the initial MOox such that the cycle can repeat. The efficiency of the process is highly dependent on the MOox/MOred pair used to facilitate the reaction. To date, the most successful demonstrations have utilized non-stoichiometric oxides, for example CeO2−δ, a special class of materials capable of continuously transitioning from MOox to MOred without changing their crystallographic phase. The success of non-stoichiometric oxides is typically attributed to their ability to maintain their crystallographic phase, allowing them to be cycled many times without destabilizing. This is a tremendous practical advantage.

In this work, we will present an alternative explanation of the success of non-stoichiometric oxides from a performance perspective. In particular, we first illuminate the importance of entropy as a key driver in the efficiency of the two-step redox cycle. The entropy change ΔSrxn of the MOox → MOred + O2 reaction is a gauge of the effect of the temperature swing between Step 1 and Step 2: the higher ΔSrxn, the lower the ΔT = Tred − Tox needed to thermodynamically switch between Step 1 and Step 2. Because sensible heating between Tred and Tox constitutes the major energy loss in the cycle, minimizing ΔT is key to maximizing efficiency. In this light, maximizing ΔSrxn can be seen as a first step towards finding an optimal redox material.

Stoichiometric most metal/oxide pairs, for example SnO2 → Sn + O2, suffer from an inherent flaw: the reduced metal is typically more ordered than its oxide counterpart. The thermodynamic significance of this is paramount. The reaction entropy is ΔSrxn = SMOred + SO2− SMOox = Sss + SO2, where Sss = SMOred− SMOox is the entropy change in the solid state and SO2 is the entropy associated with the liberated oxygen. In this reaction ΔSrxn will be a positive quantity, and here we would like to maximize it. Now SO2 is fixed by the temperature and partial pressure of the system. Holding those operating parameters constant, the only way we can increase ΔSrxn is by making Sss positive and as large as possible. However, recall that for a typical stoichiometric pair the reduced metal is more ordered (lower entropy) than its oxide. Therefore, for most stoichiometric pairs Sss is negative, and so contributes negatively to ΔSrxn.

For a non-stoichiometric pair, for example CeO2 → CeO1.8 + 0.1O2, the above analysis procedure remains the same, but the result can be starkly different. Oxygen liberation in a non-stoichiometric oxide is concomitant with oxygen vacancy formation in the crystal. Therefore, reduced non-stoichiometric oxides are less ordered than their oxidized counterpart, which can be attributed to the configurational entropy associated with vacancy formation in the lattice. Because of this, Sss can be positive for non-stoichiometric pairs, thereby giving them the unique capability of boosting to ΔSrxn above that of SO2.

In this work, the above phenomenon is investigated from an empirical and statistical thermodynamic approach. In particular, we investigate the potential to maximize ΔSrxn through the appropriate choice of non-stoichiometric material, and nonstoichiometry, δ, endpoints for both the reduction and oxidation step. We then explore how this unique property of non-stoichiometric can be exploited to maximize the theoretical conversion efficiency of two-step thermochemical water and CO2 splitting. Ultimately, we hope that this works will shed some light on the role of entropy in redox cycles based on non-stoichiometric oxides, and may pave a path to new materials discovery, guided by a principle of maximizing the solid-state entropy change.

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