When conductive solids with nanoscale porosity are immersed in electrolyte and biased against a suitable counter electrode, the surface is charged and the properties of the matter at the surface vary reversibly as the function of the bias voltage. Due to the immense surface area at nanoscale pore size, the local changes are reflected in the macroscopic materials properties. Specifically, changes in the superficial interatomic bonding lead to a stress and strain of the material, with strain amplitude and strain energy density of sufficient magnitude to suggest possible application as actuator materials. We discuss an experiment which provides an insight into the underlying microscopic processes at the metal electrolyte interface: are changes in the surface bonding in the metal dominant, or does the effect arise mainly from the formation of chemical bonds with adsorbates? We studied the variation of the surface stress charge coefficient of Pt in weakly adsorbing aqueous solutions of NaF of various concentrations. As the concentration is reduced, we find initially an increase in the magnitude of the surface stress-charge coefficient, followed by saturation at a value of -1.9V. Since specific adsorption is expected to be reduced as the solution become more diluted, in favor of the capacitive charging, the results support the notion that changes in the bonding between the metal atoms control the variation in the surface stress during double-layer charging.
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