Following the pioneering work of Vayenas and co-workers, the feasibility of electrochemical promotion of heterogeneously catalyzed reactions has been demonstrated in the past two decades with porous metal catalysts interfaced to solid electrolytes. [1][2][3] The catalytic activity of electrically polarized porous metal electrodes on solid electrolytes has been investigated in over 100 reaction systems, and up to 200-fold increases in rate have been reported. Mechanistically, the promotion effect has been attributed to a charged mobile species that is (partially) discharged at the triple-phase boundary (tpb) (solid electrolyte/metal/gas) and then diffuses over the metal surface, thereby modifying its catalytic activity. [3][4][5][6] The crucial role of the electrochemically generated spillover species and its chemical identity have been proven in recent years by surface analytical techniques for only two systems: the oxygen-ion-conducting Pt/YSZ system (YSZ = yttria-stabilized zirconia, ZrO 2 + Y 2 O 3 ) [4,[6][7][8][9] and the alkali-ion-conducting Pt/b''-Al 2 O 3 system. [10,11] We report herein the imaging of the spillover process itself. We observe the diffusional spreading of oxygen upon electrochemical pumping on a dense platinum film interfaced with YSZ. The experiments provide a better basis for a detailed mechanistic understanding of the electrochemical promotion of surface reactions.The typical cell arrangement of an electrochemical promotion experiment is shown in Figure 1 a, and the generation of spillover oxygen at the tpb is illustrated. Oxygen ions are transported through the YSZ solid electrolyte during the anodic (positive) polarization of the metal catalyst (working electrode), and the following reaction [Eq. (1)] takes place at the tpb:The atomic oxygen generated at the tpb (O tpb ) spills over the electrode surface forming a layer of chemisorbed oxygen (O ad ). It was demonstrated with planar YSZ/Pt model catalysts studied in an ultrahigh-vacuum (UHV) chamber that the electrochemically generated spillover oxygen is identical to oxygen adsorbed from the gas phase. [8,12] However, instead of the expected diffusion from the tpb, a seemingly uniform increase in the oxygen concentration across the metal surface was observed. This uniform increase in oxygen coverage could be attributed to a high density of pores (grain boundaries) in the metal film, which act as fastdiffusion paths to transport oxygen from the Pt/YSZ interface to the outer metal surface.To obtain a planar model catalyst, a pore-free, 200-to 250-nm thick Pt film was deposited by pulsed laser deposition on the (111) plane of YSZ. Subsequent annealing produced a dense Pt film with (111) surface orientation as shown in Figure 1 b. Two spatially resolving methods were applied to image the spillover process in situ. With photoelectron emission microscopy (PEEM), the photoelectrons ejected during illumination by a D 2 discharge lamp (5.5 eV-6.2 eV) are used to image the local work function (WF) with a spatial resolution of approximately 1 m...