Naghavi S Shahab, He Jiangang, Wolverton C
Department of Physical and Computational Chemistry, Shahid Beheshti University, G.C., Evin, 1983969411 Tehran, Iran.
Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States.
ACS Appl Mater Interfaces. 2020 May 13;12(19):21521-21527. doi: 10.1021/acsami.0c01083. Epub 2020 May 4.
A large entropy of reduction is crucial in achieving materials capable of high-efficiency solar thermochemical hydrogen (STCH) production through two-step thermochemical water splitting cycles. We have recently demonstrated that the onsite electronic entropy of reduction attains an extreme value of 4.26 at 1500 K in Ce → Ce redox reactions, which explains the high performance and uniqueness of CeO as an archetypal STCH material. However, ceria requires high temperatures ( > 1500 °C) to achieve a reasonable reduction extent because of its large reduction enthalpy, which is a major obstacle in practical applications. Therefore, new materials with a large entropy of reduction and lower reduction enthalpy are required. Here, we perform a systematic screening to search for Ce-based oxides which possess thermodynamics superior to CeO for STCH production. We first search the Inorganic Crystal Structure Database (ICSD) and literature for Ce-based oxides and subsequently use density functional theory to compute their reduction enthalpies (i.e., oxygen vacancy formation energies). We find that CeTiO with the brannerite structure is the most promising candidate for STCH because it possesses three essential characteristics of an STCH material: (i) a smaller reduction enthalpy compared to ceria yet large enough to split water, (ii) a high thermal stability, as reported experimentally, and (iii) a large entropy of reduction associated with Ce → Ce redox. Our proposed design strategy suggests that further exploration of Ce oxides for STCH production is warranted.
在通过两步热化学水分解循环实现能够高效太阳能热化学制氢（STCH）的材料方面，大的还原熵至关重要。我们最近证明，在Ce→Ce氧化还原反应中，原位电子还原熵在1500 K时达到4.26的极值，这解释了CeO作为典型STCH材料的高性能和独特性。然而，由于其较大的还原焓，二氧化铈需要高温（>1500°C）才能达到合理的还原程度，这是实际应用中的一个主要障碍。因此，需要具有大还原熵和较低还原焓的新材料。在这里，我们进行了系统筛选，以寻找用于STCH生产的热力学性能优于CeO的铈基氧化物。我们首先在无机晶体结构数据库（ICSD）和文献中搜索铈基氧化物，随后使用密度泛函理论计算它们的还原焓（即氧空位形成能）。我们发现具有钛铁矿结构的CeTiO是STCH最有前途的候选材料，因为它具有STCH材料的三个基本特征：（i）与二氧化铈相比还原焓较小，但大到足以分解水；（ii）如实验报道的高热稳定性；（iii）与Ce→Ce氧化还原相关的大还原熵。我们提出的设计策略表明，有必要进一步探索用于STCH生产的铈氧化物。
