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The electrocatalytic properties of advanced metal-oxide catalysts are often related to a synergistic interplay between multiple active catalyst phases. The structure and chemical nature of these active phases are typically established under reaction conditions, i.e. upon interaction of the catalyst with the electrolyte. Here, we present a fundamental surface science (scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction) and electrochemical (cyclic voltammetry) study of CeO 2 (111) nanoislands on Pt(111) in blank alkaline electrolyte (0.1 M KOH) in a potential window between -0.05 and 0.9 V RHE . We observe a size- and preparation-dependent behavior. Large ceria nanoislands prepared at high temperatures exhibit stable redox behavior with Ce 3+ /Ce 4+ electrooxidation/reduction limited to the surface only. In contrast, ceria nanoislands, smaller than ∼5 nm prepared at a lower temperature, undergo conversion into a fully hydrated phase with Ce 3+ /Ce 4+ redox transitions, which are extended to the subsurface region. While the formation of adsorbed OH species on Pt depends strongly on the ceria coverage, the formation of adsorbed H ads on Pt is independent of the ceria coverage. We assign this observation to intercalation of H ads at the Pt/ceria interface. The intercalated H ads cannot participate in the hydrogen evolution reaction, resulting in the moderation of this reaction by ceria nanoparticles on Pt.

The development of robust electrocatalysts with low platinum content for acidic hydrogen evolution reaction (HER) is paramount for large scale commercialization of proton exchange membrane electrolyzers. Herein, a simple strategy is reported to synthesize a well anchored, low Pt containing Vulcan carbon catalyst using ZnO as a sacrificial template. Pt containing ZnO (PZ) is prepared by a simultaneous borohydride reduction. PZ is then loaded onto Vulcan carbon to produce a very low Pt content electrocatalyst, PZ@VC. PZ@VC with 2 wt.% Pt shows excellent performance for acidic HER in comparison to the commercial Pt/C (20 wt.%) catalyst. PZ@VC with a very low Pt loading shows significantly low η 10 and η 100 values (15 and 46 mV, respectively). PZ@VC on coating with Nafion (PZ@VC-N) shows further improvement in its performance (η 10 of 7 mV, η 100 of 28 mV) with ≈300 h of stability (≈10 mA cm -2 ) with only 4 µg Pt cm -2 . PZ@VC-N shows a record high mass activity of 71 A mg Pt -1 (32 times larger than Pt/C (20 wt.%) at 50 mV of overpotential. Post reaction characterizations reveal Pt nanoparticles are embedded onto VC with no traces of zinc, suggestive of a strong metal-support interaction leading to this high stability at low Pt loading., (© 2023 Wiley-VCH GmbH.)