Facet-Resolved Electric Double Layer of Polycrystalline Gold.

In electrochemistry, polycrystalline gold is widely used as a practical and model electrode because of its relative inertness, yet interpreting its electric double-layer (EDL) response remains nontrivial. In practice, this response is often reduced to a single feature, a capacitance minimum near the potential of zero charge (PZC) of Au(110), which has long been taken as evidence that polycrystalline Au is dominated by its least dense facet. Here we systematically demonstrate that this apparent dominance reflects a masking effect, not an intrinsic absence of other facet contributions. Using electron backscatter diffraction (EBSD)-characterized polycrystalline electrodes, single-crystal facets, and a cylindrical electrode with a basal Au(111) facet and polycrystalline sidewalls, we resolve two and even three distinct minima on the same electrode at potentials characteristic of the PZC of Au(111), Au(100), and Au(110). This provides direct evidence that, in conventional polycrystals, the (111) and (100) contributions, long hypothesized, are present but are typically hidden by the strongly potential-dependent (110) response. We further show that stepped surfaces, such as Au(554), cannot be captured by the simple additive model of the parent low-index facets, as terraces and step edges carry distinct local charge densities that are screened collectively rather than independently. Grain boundaries, by contrast, exert little measurable influence in our experiments, consistent with their low areal density. Together, these findings establish a broader principle: polycrystalline Au electrodes may appear dominated by a single facet, but the measured capacitance curve reflects the sum of contributions from multiple facets, whose visibility depends on facet domain size and Debye screening length.