Molecular Insight in Structure and Activity of Highly Efficient, Low-Ir Ir–Ni Oxide Catalysts for Electrochemical Water Splitting (OER)

Abstract

Mixed bimetallic oxides offer great opportunities for a systematic tuning of electrocatalytic activity and stability. Here, we demonstrate the power of this strategy using well-defined thermally prepared Ir-Ni mixed oxide thin film catalysts for the electrochemical oxygen evolution reaction (OER) under highly corrosive conditions such as in acidic proton exchange membrane (PEM) electrolyzers and photoelectrochemical cells (PEC). Variation of the Ir to Ni ratio resulted in a volcano type OER activity curve with an unprecedented 20-fold improvement in Ir mass-based activity over pure Ir oxide. In situ spectroscopic probing of metal dissolution indicated that, against common views, activity and stability are not directly anticorrelated. To uncover activity and stability controlling parameters, the Ir-Ni mixed thin oxide film catalysts were characterized by a wide array of spectroscopic, microscopic, scattering, and electrochemical techniques in conjunction with DFT theoretical computations. By means of an intuitive model for the formation of the catalytically active state of the bimetallic Ir-Ni oxide surface, we identify the coverage of reactive surface hydroxyl groups as a suitable descriptor for the OER activity and relate it to controllable synthetic parameters. Overall, our study highlights a novel, highly active oxygen evolution catalyst; moreover, it provides novel important insights into the structure and performance of bimetallic oxide OER electrocatalysts in corrosive acidic environments.

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

“…During both potential sweep and steady-state polarization regimes, the rate of Sn dissolution significantly exceeds the rate of Ir dissolution. A similar trend in dissolution was also observed in the case of Ir-Ru and Ir-Ni mixed oxides [10,14,34]. The onset of Ir dissolution is ca.…”

“…There are several options given in the literature for how this can be achieved. Among them, the increase in surface area of Ir-based catalysts by using nanoparticles [8] or porous structures [9,10] is the most used approach for improving the gravimetric electrocatalytic activity. Another possibility is dilution of Ir by alloying with other metals.…”

“…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.…”

MAXNET Energy – Focusing Research in Chemical Energy Conversion on the Electrocatlytic Oxygen Evolution

“…Once again, however, there are deviations from the activity-stability counterbalance, calling for a more detailed investigation [44]. In a very recent work of Reier et al [45] the authors suggested that activity and stability of Ir may be related to the coverage of reactive surface hydroxyl groups. The latter can be controlled during synthesis and/or post-annealing.…”