Tan Wenzhou, Huan Daoming, Yang Wenqiang, Shi Nai, Wang Wanhua, Peng Ranran, Wu Xiaojun, Lu Yalin
CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China Hefei 230026 Anhui China
Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China Hefei Anhui 230026 China.
RSC Adv. 2018 Jul 25;8(47):26448-26460. doi: 10.1039/c8ra04059a. eCollection 2018 Jul 24.
Exploring mechanisms for sluggish cathode reactions is of great importance for solid oxide fuel cells (SOFCs), which will benefit the development of suitable cathode materials and then accelerate cathode reaction rates. Moreover, possible reaction mechanisms for one cathode should be different when operating in oxygen ion conducting SOFCs (O-SOFC) and in proton conducting SOFCs (P-SOFCs), and therefore, they lead to different reaction rates. In this work, a Ruddlesden-Popper (R-P) oxide, SrFeO (SFO), was selected as a promising cathode for both O-SOFCs and P-SOFCs. Using the first-principles approach, a microscopic understanding of the O reactions over this cathode surface was investigated operating in both cells. Compared with LaSrCoFeO (LSCF), the low formation energies of oxygen vacancies and low migration energy barriers for oxygen ions in SFO make oxygen conduction more preferable which is essential for cathode reactions in O-SOFCs. Nevertheless, a large energy barrier (2.28 eV) is predicted for oxygen dissociation reaction over the SFO (001) surface, while there is a zero barrier over the LSCF (001) surface. This result clearly indicates that SFO shows a weaker activity toward the oxygen reduction, which may be due to the low surface energies and the specific R-P structure. Interestingly, in P-SOFCs, the presence of protons on the SFO (001) surface can largely depress the energy barriers to around 1.46-1.58 eV. Moreover, surface protons benefit the oxygen adsorption and dissociation over the SFO (001) surface. This result together with the extremely low formation energies and migration energy barriers for protons seem to suggest that SFO could work more effectively in P-SOFCs than in O-SOFCs. It's also suggested that too many protons at the SFO surface will lead to high energy barriers for the water formation process, and thus that over-ranging steam concentrations in the testing atmosphere may have a negative effect on cell performances. Our study firstly and clearly presents the different energy barriers for one cathode performing in both O- and P-SOFCs according to their different working mechanisms. The results will be helpful to find the constraints for using cathodes toward oxygen reduction reactions, and to develop effective oxide cathode materials for SOFCs.
探索阴极反应迟缓的机制对于固体氧化物燃料电池（SOFC）至关重要，这将有利于开发合适的阴极材料，进而加快阴极反应速率。此外，同一阴极在氧离子传导型SOFC（O-SOFC）和质子传导型SOFC（P-SOFC）中运行时，其可能的反应机制应有所不同，因此会导致不同的反应速率。在这项工作中，一种Ruddlesden-Popper（R-P）氧化物SrFeO（SFO）被选为O-SOFC和P-SOFC都颇具前景的阴极材料。采用第一性原理方法，研究了在这两种电池中运行时该阴极表面上氧反应的微观情况。与LaSrCoFeO（LSCF）相比，SFO中氧空位的低形成能和氧离子的低迁移能垒使得氧传导更具优势，这对于O-SOFC中的阴极反应至关重要。然而，预测SFO（001）表面上氧解离反应的能垒较大（2.28 eV），而LSCF（001）表面上的能垒为零。这一结果清楚地表明SFO对氧还原的活性较弱，这可能是由于其低表面能和特定的R-P结构所致。有趣的是，在P-SOFC中，SFO（001）表面上质子的存在可将能垒大幅降低至约1.46 - 1.58 eV。此外，表面质子有利于SFO（001）表面上的氧吸附和解离。这一结果连同质子极低的形成能和迁移能垒似乎表明SFO在P-SOFC中比在O-SOFC中能更有效地工作。还表明SFO表面过多的质子会导致水形成过程的能垒较高，因此测试气氛中过高的蒸汽浓度可能会对电池性能产生负面影响。我们的研究首次且清晰地展示了同一阴极在O-SOFC和P-SOFC中因其不同工作机制而具有不同的能垒。这些结果将有助于找出使用阴极进行氧还原反应的限制因素，并开发用于SOFC的有效氧化物阴极材料。
