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Measurement and prediction of nonlinear harmonics as a tool for dynamic characterization of electrochemical systems.

机译：非线性谐波的测量和预测作为电化学系统动态表征的工具。

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Mixed ionic and electronic conducting materials (MIECs) comprise the active catalyst in solid oxide fuel cell cathodes, ion transport membranes, and electrically-driven air separation devices. Of particular interest is the mechanism of oxygen reduction and transport pathways in these materials. Understanding these processes is difficult owing to the coupled nature of transport and kinetics in MIECs, and to the inherent limitations of linearized (near equilibrium) measurement techniques. This work presents the development of a nonlinear harmonic measurement technique to investigate these processes involving detection of higher order (nonlinear) harmonics generated by moderate amplitude AC electrochemical perturbations, and comparison of the nonlinear behavior to mathematical models of the physical phenomena governing these materials. Employing Fast Fourier Transforms, this method isolates the linear and nonlinear harmonics to specific frequency bands, and was first validated by application to the rotating disk electrode system, where the measured current harmonics were shown to agree well with theory.; A dense thin film La0.6Sr0.4CoO3-delta (LSC-64) electrode with a thickness of 900 nm, was fabricated to isolate kinetic phenomena. By comparing experimentally measured nonlinear harmonics on the LSC-64 thin film at high temperatures (> 600°C) to predictions from models that adopt various oxygen reduction scenarios, we identified oxygen dissociative adsorption as the rate limiting step in this reaction. This means that oxygen physically adsorbs onto electrode surface, and the reaction is limited by the availability of a second vacancy adjacent to the unstable physically adsorbed intermediate. The detected harmonics were very small (1% of linear response), and were naturally isolated to appropriate frequency bands with this harmonic technique. This nonlinear behavior would be difficult to distinguish from drift or noise with other techniques. LSC-64 and LSC-82 electrodes with more complex porous microstructures were also examined at high temperatures (> 600°C) to probe transport phenomena. However, disagreement between experiment and the response predicted from a one-dimensional macrohomogeneous porous electrode model was observed. This disagreement is likely due to limitations of the one-dimensional model, or a co-limiting step involved in the oxygen reduction reaction.
机译：离子和电子混合导电材料（MIEC）包括固体氧化物燃料电池阴极，离子传输膜和电动空气分离装置中的活性催化剂。特别令人感兴趣的是这些材料中的氧还原和运输途径的机理。由于MIEC中传输和动力学的耦合性质以及线性化（接近平衡）测量技术的固有局限性，因此难以理解这些过程。这项工作提出了一种非线性谐波测量技术的发展，以研究这些过程，包括检测由中等幅度AC电化学扰动产生的高阶（非线性）谐波，并将非线性行为与控制这些材料的物理现象的数学模型进行比较。该方法采用快速傅立叶变换，将线性和非线性谐波隔离到特定频带，并首先通过应用于旋转盘电极系统进行了验证，该方法显示出所测得的电流谐波与理论吻合良好。制作了厚度为900 nm的致密薄膜La0.6Sr0.4CoO3-delta（LSC-64）电极，以隔离动力学现象。通过将在高温（> 600°C）下在LSC-64薄膜上实验测量的非线性谐波与采用各种减氧方案的模型的预测进行比较，我们确定了氧离解吸附是该反应中的限速步骤。这意味着氧物理吸附在电极表面上，并且反应受到与不稳定的物理吸附中间体相邻的第二空位的可用性的限制。所检测到的谐波非常小（<线性响应的<1％），并且通过此谐波技术可以自然地隔离到适当的频带。使用其他技术很难将这种非线性行为与漂移或噪声区分开。还在高温（> 600°C）下检查了具有更复杂的多孔微结构的LSC-64和LSC-82电极，以探查传输现象。但是，观察到实验与从一维宏观均匀多孔电极模型预测的响应之间存在分歧。这种分歧可能是由于一维模型的局限性，或者是氧还原反应中涉及的共限制步骤。

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