Impact of uncompensated resistance on Hads-terminated Pt electrodeposition

Abstract

Self-terminated Pt electrodeposition on Au occurs at large negative overpotentials where hydrogen adsorption H_ads inhibits the coordination of PtCl₄²⁻ and/or PtCl₃(H₂O)⁻ to the electrode surface in chloride-supported electrolytes. Potential control can be used to toggle the H_ads coverage to enable multicycle Pt deposition. Specifically, the applied potential is stepped between + 0.4 V_SSCE and − 0.8 V_SSCE, transiting the regime of overpotential activated Pt electrodeposition. The amount of metal deposited depends on capacitive charging delays associated with the double layer and competitive Cl⁻, H, and PtCl_4-x(H₂O)_x^−2+x adsorption. In addition, significant potential deviations arise from ohmic losses that are a function of the supporting electrolyte, cell geometry, and PtCl_4-x(H₂O)_x^−2+x concentration. Taken in combination, the delay in reaching the growth termination potential leads to additional metal deposition and roughening per pulse cycle. Experiments with a parallel plate cell enable the resistive component of the ohmic losses to be specified by the separation between the working and reference electrodes. During multicycle deposition, the H_upd pseudo-capacitance associated with Pt surface sites leads to further RC time constant delays and roughening. The transition to three-dimensional growth leads to low-density films as clearly evidenced after 50 deposition cycles. The difficulties with the pulsed potential scheme can be circumvented, or at least minimized, by using electrolyte exchange to introduce the PtCl_4-x(H₂O)_x^−2+x reactant at a fixed potential, i.e., − 0.8 V_SSCE into the weakly acidic electrolyte. The resulting fractional Pt coverage per cycle is a monotonic function of K₂PtCl₄ concentration and ranged from 0.2 to almost a complete monolayer reflecting the competition between PtCl_4-x(H₂O)_x^−2+x reduction and adsorption of the blocking H_ads layer.