Controlling the double layer of platinum by selective passivation of step sites using adatom modification

Fröhlich, N.L.; Hu, Y.; Larsson, A.L.; Koper, M.T.M.

doi:doi:10.1021/jacs.6c07209

The structure of the electric double layer at platinum electrodes remains incompletely understood, even for the model Pt(111)/HClO₄ interface, which deviates significantly from Gouy–Chapman–Stern theory. While Pt(111) exhibits a true double-layer window (0.40–0.60 V_RHE) that enables direct measurement of the double-layer capacitance, stepped Pt surfaces do not because hydrogen and/or hydroxyl species adsorb at low-coordinated step sites across the entire potential range. We previously showed that hydroxyl adsorption on (110)-steps is potential-independent within this nominal double-layer window, leading to decreasing capacitance with increasing (110)-step density due to suppression of the step Helmholtz capacitance. In contrast, (100)-steps exhibit potential-dependent hydroxyl adsorption that introduces a substantial pseudocapacitive contribution and increases capacitance with step density. Here, we selectively passivate Pt step sites by depositing Au* and...Show moreThe structure of the electric double layer at platinum electrodes remains incompletely understood, even for the model Pt(111)/HClO₄ interface, which deviates significantly from Gouy–Chapman–Stern theory. While Pt(111) exhibits a true double-layer window (0.40–0.60 V_RHE) that enables direct measurement of the double-layer capacitance, stepped Pt surfaces do not because hydrogen and/or hydroxyl species adsorb at low-coordinated step sites across the entire potential range. We previously showed that hydroxyl adsorption on (110)-steps is potential-independent within this nominal double-layer window, leading to decreasing capacitance with increasing (110)-step density due to suppression of the step Helmholtz capacitance. In contrast, (100)-steps exhibit potential-dependent hydroxyl adsorption that introduces a substantial pseudocapacitive contribution and increases capacitance with step density. Here, we selectively passivate Pt step sites by depositing Au* and Ag* adatoms. We find that Au*_step-modification suppresses step-specific adsorption, restoring predominantly electrostatic behavior for (100)-type stepped Pt surfaces and reversing the capacitance trends observed for the bare stepped surfaces. In contrast, Ag*_step-modification introduced an additional chemical contribution, manifested as substantially increased capacitance and enhanced CO oxidation activity due to adsorption of oxophilic species on Ag*. These results demonstrate that Pt step-site chemistry, and consequently the electrical double-layer structure and electrocatalytic activities, can be tuned and probed to a remarkable degree of controllability through selective adatom modification.
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Controlling the double layer of platinum by selective passivation of step sites using adatom modification

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