On the Origin of the Improved Ruthenium Stability in RuO₂–IrO₂Mixed Oxides

Abstract

High oxygen evolution reaction activity of ruthenium and long term stability of iridium in acidic electrolytes make their mixed oxides attractive candidates for utilization as anodes in water electrolyzers. Indeed, such materials were addressed in numerous previous studies. The application of a scanning flow cell connected to an inductively coupled plasma mass spectrometer allowed us now to examine the stability and activity toward oxygen evolution reaction of such mixed oxides in parallel. The whole composition range of Ir-Ru mixtures has been covered in a thin film material library. In the whole composition range the rate of Ru dissolution is observed to be much higher than that of Ir. Eventually, due to the loss of Ru, the activity of the mixed oxides approaches the value corresponding to pure IrO 2 . Interestingly, the loss of only a few percent of a monolayer in Ru surface concentration results in a significant drop in activity. Several explanations of this phenomenon are discussed. It is concluded that the herein observed stability of mixed Ir-Ru oxide systems is most likely a result of high corrosion resistance of the iridium component, but not due to an alteration of the material's electronic structure. Renewable primary energies such as solar energy, wind energy and ocean energy receive more and more attention and are increasingly installed around the world.1-3 It is anticipated that renewables will eventually replace traditional fossil fuel-burning and nuclear power plants. However, intermittent power supply of renewables means that energy needs to be buffered. Thereby, hydrogen produced by water electrolysis is considered as an ideal energy carrier to adjust the balance between the generation of power by renewable primary energy and energy demand for end-use.3-5 Currently, acidic proton exchange membrane water electrolysis (PEMWE) is considered as a promising technology for this purpose. However, the widespread use of PEMWE is hindered by high capital costs, low efficiency, and shortages related to performance deterioration with time. 6 In this connection the nature of electrocatalysts and the procedure of their production and application conditions play a critical role. Materials used as electrocatalysts must be as active as possible to improve efficiency, while at the same time they need to be stable to maintain this efficiency throughout the lifetime of the electrolyzer. This is especially critical for materials catalyzing the anodic oxygen evolution reaction (OER), because of the detrimental positive potential and highly corrosive acidic environment. Only a few catalysts are able to withstand these harsh conditions, while providing sufficient activity, conductivity and mechanical stability. In fact, only iridium oxide anodes are proven to provide the required longevity of operation. On the other hand, ruthenium shows the highest electrocatalytic activity toward this reaction. 7,8 During the last decades, the electrochemical and surface properties of anodes based on these metals and their oxides we...

Using Instability of a Non-stoichiometric Mixed Oxide Oxygen Evolution Catalyst As a Tool to Improve Its Electrocatalytic Performance

“…XPS has been effectively applied as both an ex-situ and in-situ surface-sensitive technique in numerous studies of pristine or mixed Ir-based oxides for OER [12,14,25,28,29]. It provides information about the chemical state and ratio between elements in as-prepared materials and even more importantly on the change in the composition during anodic polarization.…”

“…Another possibility is dilution of Ir by alloying with other metals. The representative examples here are the systems in which Ir is alloyed with electrochemically less stable metals, e.g., Ir-Ru [11][12][13][14] and IrNi mixed oxide [10] or Ir-based perovskites [15,16], which sometimes show higher activity than Ir oxide. As a rule, however, improved electrocatalytic activity of these compounds is counterbalanced by their lower stability towards dissolution.…”

“…As a rule, however, improved electrocatalytic activity of these compounds is counterbalanced by their lower stability towards dissolution. In these systems, dissolution of the less stable component usually occurs together with an increased dissolution of Ir, resulting in the overall catalyst instability [10,14]. On the other hand, mixing of Ir oxide with significantly less active but more stable materials, e.g., Ti [17] or Sn [18] typically results in catalysts showing very high stability against dissolution.…”

Screening Bifunctional Pt Based NSTF Catalysts for Durability with the Rotating Disk Electrode: The Effect of Ir and Ru

“…Ru is thus often mixed with Ir to balance the activity, stability and cost properties of an OER-in-acid catalyst. [8][9][10][11][12][13][14] Therefore the addition of Ru to Ir is performed in this study with the hope of finding similarly durable catalysts with lower Ir loadings. Further motivation to include Ru in this study is the strong possibility that the NSTF support has a stabilizing effect on Ru.…”

“…Figure 1 shows the deposition scheme. It should be stated that the Ir and/or Ru on Pt catalyst material was not deposited as films, as Figure 1 may seem to suggest, but more akin to nanoparticles because of the high roughness factor (10)(11)(12)(13)(14)(15) 21 of NSTF.…”