The low efficiency of the electrocatalytic oxidation of water to O2 (oxygen evolution reaction‐OER) is considered as one of the major roadblocks for the storage of electricity from renewable sources in form of molecular fuels like H2 or hydrocarbons. Especially in acidic environments, compatible with the powerful proton exchange membrane (PEM), an earth‐abundant OER catalyst that combines high activity and high stability is still unknown. Current PEM‐compatible OER catalysts still rely mostly on Ir and/or Ru as active components, which are both very scarce elements of the platinum group. Hence, the Ir and/or Ru amount in OER catalysts has to be strictly minimized. Unfortunately, the OER mechanism, which is the most powerful tool for OER catalyst optimization, still remains unclear. In this review, we first summarize the current state of our understanding of the OER mechanism on PEM‐compatible heterogeneous electrocatalysts, before we compare and contrast that to the OER mechanism on homogenous catalysts. Thereafter, an overview over monometallic OER catalysts is provided to obtain insights into structure‐function relations followed by a review of current material optimization concepts and support materials. Moreover, missing links required to complete the mechanistic picture as well as the most promising material optimization concepts are pointed out.

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.

Redox-active support materials can help reduce the noble-metal loading of a solid chemical catalyst while offering electronic catalyst-support interactions beneficial for catalyst durability. This is well known in heterogeneous gas-phase catalysis but much less discussed for electrocatalysis at electrified liquid-solid interfaces. Here, we demonstrate experimental evidence for electronic catalyst-support interactions in electrochemical environments and study their role and contribution to the corrosion stability of catalyst/support couples. Electrochemically oxidized Ir oxide nanoparticles, supported on high surface area carbons and oxides, were selected as model catalyst/support systems for the electrocatalytic oxygen evolution reaction (OER). First, the electronic, chemical, and structural state of the catalyst/support couple was compared using XANES, EXAFS, TEM, and depth-resolved XPS. While carbon-supported oxidized Ir particle showed exclusively the redox state (+4), the Ir/IrOx/ATO system exhibited evidence of metal/metal-oxide support interactions (MMOSI) that stabilized the metal particles on antimony-doped tin oxide (ATO) in sustained lower Ir oxidation states (Ir(3.2+)). At the same time, the growth of higher valent Ir oxide layers that compromise catalyst stability was suppressed. Then the electrochemical stability and the charge-transfer kinetics of the electrocatalysts were evaluated under constant current and constant potential conditions, where the analysis of the metal dissolution confirmed that the ATO support mitigates Ir(z+) dissolution thanks to a stronger MMOSI effect. Our findings raise the possibility that MMOSI effects in electrochemistry-largely neglected in the past-may be more important for a detailed understanding of the durability of oxide-supported nanoparticle OER catalysts than previously thought.

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