Palladium (Pd) is an active catalyst for various reactions, such as hydrogen evolution (HER) and hydrogen oxidation (HOR) reactions. However, its activity can be further optimized by introducing strain and ligand effects from Pd deposition onto suitable substrates like gold (Au). In this study, we use scanning electrochemical microscopy (SECM) to investigate the catalytic properties of such Pd/Au systems. For the HER, a sub-monolayer of Pd (PdML) was electrochemically deposited onto half of a polycrystalline (pc) Au substrate with underpotential deposition (UPD). The localized activity measurements revealed improved HER kinetics for Pd atoms at the Pd/Au border in 0.1 M HClO4. As a consequence, a set of Pd/Au samples with increasing density of Pd/Au borders was synthesized by atomic layer deposition (ALD). These ALD Pd deposits have an increased thickness compared to a sub-monolayer, which makes hydride formation thermodynamically viable. Because of this, the samples were investigated for the HOR/H absorption activity using the redox competition (RC) mode. We highlight the influence of cations in 0.1 M AMOH (AM = Li+, Na+, K+, Rb+, Cs+) electrolytes on the HOR/H absorption activity, displaying higher activities for larger cations: jLiOH < jNaOH < jKOH < jRbOH < jCsOH. From the spatial and temporal resolution of the activity, active spots are identified, which expand with time and diminishing hydrogen concentration in the electrolyte. Additional laser-induced current transient (LICT) experiments confirm the dependency between cation and electrocatalytic activity observed with RC-SECM.

Catalysis Research Center TUM Ernst Otto Fischer Str 1 Garching 85748 Germany

Center of Materials and Nanotechnologies Faculty of Chemical Technology University of Pardubice Nam Cs Legii 565 Pardubice 53002 Czech Republic

Central European Institute of Technology Brno University of Technology Purkynova 123 Brno 61200 Czech Republic

Department of Physical Chemistry and Modelling Faculty of Materials Science and Ceramics AGH University of Science and Technology Al Mickiewicza 30 Kraków 30 059 Poland

Physics of Energy Conversion and Storage Technical University of Munich James Franck Str 1 Garching 85748 Germany

Research Group of Battery Materials and Technologies Department of Mechanical and Materials Engineering Faculty of Technology University of Turku Turun Yliopisto 20014 Finland

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