Nenning Andreas, Breitwieser Stanislaus, Melcher Christian, Fleig Jürgen
Institute of Chemical Technologies and Analytics, Research Group for Electrochemical Energy Conversion TU Wien Austria
J Mater Chem A Mater. 2025 Jul 31. doi: 10.1039/d5ta02648b.
The (electro)chemical properties of electrode materials in solid oxide cells or oxide-based redox catalysts are determined by the surface chemistry of these materials under operation conditions. Surface point defect concentrations strongly depend on the oxygen stoichiometry in the bulk and the gas phase's chemical composition (, oxygen activity). However, many chemically sensitive surface analysis techniques rely on UHV conditions, leading to a two-fold deviation from surfaces under operational conditions. On the one hand, adsorbed gas phase species are missing in UHV. On the other hand, transition metal oxidation states and the oxygen vacancy concentration at surfaces are connected to the oxygen stoichiometry in the bulk of the material, which is inevitably altered during cell transfer from electrochemical measurement to UHV-based analytics. To reduce this two-fold gap between analytical studies and typical operation conditions, we present a novel solid oxide cell design for electrochemical oxygen activity control of surfaces in UHV-based analytic tools. Its key feature is an oxygen-ion buffering counter electrode containing a Fe|FeO phase equilibrium with known oxygen activity. A defined voltage between this counter electrode and the oxide under investigation (used as working electrode) defines the oxygen activity of the relevant oxide surface. Moreover, simultaneous thin film coulometry allows the determination of the bulk oxygen deficiency in the respective oxides. As a proof of concept, we use UHV-based XPS to compare the bulk and surface reducibility of fluorite-type Gd-doped ceria and perovskite-type Fe-doped SrTiO under electrochemical oxygen activity control. We show that the cell voltage can tune the transition metal oxidation states and oxygen vacancy concentration at the surface. These relate well to the actual solid oxide cell operation at the same temperature and (O).
固体氧化物电池或氧化物基氧化还原催化剂中电极材料的（电）化学性质由这些材料在运行条件下的表面化学决定。表面点缺陷浓度强烈依赖于体相中的氧化学计量和气相的化学成分（即氧活度）。然而，许多化学敏感表面分析技术依赖于超高真空条件，导致与运行条件下的表面有两方面的偏差。一方面，超高真空中缺少吸附的气相物种。另一方面，表面的过渡金属氧化态和氧空位浓度与材料体相中的氧化学计量相关，而在从电化学测量转移到基于超高真空的分析过程中，材料体相中的氧化学计量不可避免地会发生改变。为了缩小分析研究与典型运行条件之间的这两方面差距，我们提出了一种新颖的固体氧化物电池设计，用于在基于超高真空的分析工具中对表面进行电化学氧活度控制。其关键特性是一个氧离子缓冲对电极，其中包含具有已知氧活度的Fe|FeO相平衡。该对电极与被研究氧化物（用作工作电极）之间的特定电压定义了相关氧化物表面的氧活度。此外，同步薄膜库仑法能够测定各氧化物中的体相氧缺陷。作为概念验证，我们使用基于超高真空的XPS来比较在电化学氧活度控制下萤石型钆掺杂二氧化铈和钙钛矿型铁掺杂钛酸锶的体相和表面还原性。我们表明，电池电压可以调节表面的过渡金属氧化态和氧空位浓度。这些与在相同温度和（氧）条件下实际的固体氧化物电池运行情况密切相关。
