Metals and alloys under highly oxidizing conditions: in situ studies for elucidation of “transpassive” processes

在电化学过程的许多领域中，最不了解的电位范围之一是所谓的钝化金属的高阳极（氧化）电位。在本质上，三个过程可以发生在这个潜在的制度：金属溶解，氧化膜的形成，和氧气的演变（从水氧化）。该项目的目标是详细研究选定的与技术有关的钝化金属和合金在过钝化区的电化学行为和机制。了解这些过程对于金属材料在许多应用中的可持续性能至关重要，在这些应用中，钝化合金可能暴露于高氧化条件下，例如在电解槽中用作双极板。尽管过钝性现象早已被认识，但在基本理解和所涉及的反应途径的贡献方面仍存在许多悬而未决的问题。这是由于在过钝化条件下发生的可能的电极反应的复杂性质，因为它们通常涉及固态氧化物形成、溶解（形成溶剂化的金属阳离子）和气体形成。所有这些反应取决于材料/电解质系统，是潜在的和时间依赖性的，并经常发生在动态平衡条件下的过程重叠，因此，在原位（分析）技术是最需要的反应机理的阐明。传统的反应性研究主要依赖于监测电化学行为，这不能检测非法拉第副反应或区分不同的重叠阳极氧化反应。在目前的项目中，一个独特的，原位呼吸测定技术将与电化学，结合后极化表征的材料表面。通过这种方法，我们的目标是实现一个更普遍的理解的关键因素，在过钝反应，以及金属溶解，氧化物形成和氧气的演变的相互作用。在实际应用方面，我们将研究技术上感兴趣的金属和合金，包括模型系统，如合金成分的纯金属和二元模型合金。三组材料将是感兴趣的：i）金属和合金，其已知的过钝化溶解源自钝化膜氧化成可溶性高价物质; ii）其他钝化系统，其能够使水氧化反应在表面上发生（电子传导钝化膜）; iii）阀金属。这些数据被汇集在一起，以桥接氧化物层的性质（其离子和电子性质，溶解度）与水氧化表面的催化性质。 这种理解应有助于开发设计策略的金属材料在水溶液中的高阳极电位制度的具体应用。

One of the least understood potential ranges in many fields of electrochemical processes is at highly anodic (oxidizing) potentials of so-called passive metals. In essence, three processes can take place in this potential regime: metal dissolution, oxide film formation, and oxygen evolution (from water oxidation). The project targets detailed studies of electrochemical behaviour and mechanisms in the transpassive region of selected technologically relevant passive metals and alloys. An understanding of these processes is essential for a sustainable performance of metallic materials in many applications where passive alloys may be exposed to highly oxidizing conditions, such as in their use as bipolar plates in electrolyzers. Even though the phenomenon of transpassivity has been long recognized, many open questions exist in fundamental understanding and the contribution of involved reaction pathways. This is due to the complex nature of possible electrode reactions taking place under transpassive conditions, as they often involve solid state oxide formation, dissolution (formation of solvatized metal cations) and gas formation. All these reactions depend on the material/electrolyte system, are potential- and time-dependent, and often occur in overlap of processes under dynamic equilibrium conditions; therefore in situ (analytical) techniques are most desired for elucidation of reaction mechanisms. Conventional investigations on reactivity rely mainly on monitoring the electrochemical behaviour which is not able to detect non-faradaic side reactions or to distinguish between different overlapping anodic oxidation reactions. In the current project a unique, in situ respirometry technique will be coupled with electrochemistry, combined with post-polarization characterization of the material surfaces. With this approach, we aim to achieve a more universal understanding on the critical factors in transpassive reactivity, and on the interplay of metal dissolution, oxide formation and oxygen evolution. In view of practical applications we will investigate technologically interesting metals and alloys, including model systems such as pure metals of the constituents of the alloys and binary model alloys. Three groups of materials will be of interest: i) metals and alloys with known transpassive dissolution originating from the oxidation of the passivating film into soluble higher-valent species; ii) other passive systems enabling water oxidation reaction to take place on the surface (electron conductive passive films); iii) valve metals. The data is brought together to bridge the nature of the oxide layer (its ionic and electronic properties, solubility) to the catalytic nature of the surface for water oxidation. This understanding should help to develop design strategies for specific applications of metallic materials in the high anodic potential regime in aqueous solutions.