Effect of cation species on surface-induced phase transition observed for platinum complex anions in platinum electrodeposition using nanoporous silicon

Abstract

In an earlier work [K. Fukami et al., J. Chem. Phys. 138, 094702 (2013)], we reported a transition phenomenon observed for platinum complex anions in our platinum electrodeposition experiment using nanoporous silicon. The pore wall surface of the silicon electrode was made hydrophobic by covering it with organic molecules. The anions are only weakly hydrated due to their large size and excluded from the bulk aqueous solution to the hydrophobic surface. When the anion concentration in the bulk was gradually increased, at a threshold the deposition behavior exhibited a sudden change, leading to drastic acceleration of the electrochemical deposition. It was shown that this change originates from a surface-induced phase transition: The space within a nanopore is abruptly filled with the second phase in which the anion concentration is orders of magnitude higher than that in the bulk. Here we examine how the platinum electrodeposition behavior is affected by the cation species coexisting with the anions. We compare the experimental results obtained using three different cation species: K(+), (CH3)4N(+), and (C2H5)4N(+). One of the cation species coexists with platinum complex anions [PtCl4](2-). It is shown that the threshold concentration, beyond which the electrochemical deposition within nanopores is drastically accelerated, is considerably dependent on the cation species. The threshold concentration becomes lower as the cation size increases. Our theoretical analysis suggests that not only the anions but also the cations are remarkably enriched in the second phase. The remarkable enrichment of the anions alone would give rise to the energetic instability due to electrostatic repulsive interactions among the anions. We argue that the result obtained cannot be elucidated by the prevailing view based on classical electrochemistry. It is necessitated to consult a statistical-mechanical theory of confined aqueous solutions using a molecular model for water.

High-Rate Charging of Zinc Anodes Achieved by Tuning Hydration Properties of Zinc Complexes in Water Confined within Nanopores

“…On the other hand, the SIFT that occurs during metal electrodeposition within nanoporous electrode does explain the deposition behavior in the nanopores. 21,22 The enrichment of reactants resulting from SIFT is known to promote dense metal deposition within porous silicon electrodes, because large metal ions that are weakly hydrated also behave as rather hydrophobic solutes. 21,22 Although the zerovalent zinc complexes are predicted to be slightly polarized, these complexes exhibit weaker hydration properties than the ionic species.…”

“…The deposition rate required for metallic zinc considered in the present study is much higher than that actually occurring for platinum − mentioned above. Theoretically, under the conditions of SIFT, fast supply of zinc precursors into the nanopores can be realized and the depletion does not occur even when they are rapidly consumed within the nanopores due to high-rate charging.…”

Mechanism of Accelerated Zinc Electrodeposition in Confined Nanopores, Revealed by X-ray Absorption Fine Structure Spectroscopy

“…At the single-digit nanometer scale, recent research has revealed that the conventional strategy employing hydrophilic treatment is no longer the optimum method. Because interactions with the solvent (water molecules in an aqueous solution) in deposition baths are not negligible at the nanometer scale, some studies, including our previous report, have shown that the solvation properties (hydrophobicity in an aqueous solution, solvophobicity in nonaqueous solution, and ionophobicity in an ionic liquid) of the solute and surface play significant roles in enhancing electrochemical properties such as in electrodeposition and electric double layer capacitors. − …”

“…10,13,14 Theoretical analysis based on integral equation theory for molecular liquids, a statistical−mechanical theory, suggested that the number density of metal complexes can be orders of magnitude higher with the occurrence of the SIFT in nanopores. 8,9 There exists a strong relationship between the occurrence of SIFT in nanopores and the local enhancement of electrodeposition. However, we have not obtained clear and direct evidence of the accumulation by SIFT.…”

Accelerated growth from amorphous clusters to metallic nanoparticles observed in electrochemical deposition of platinum within nanopores of porous silicon

“…When using a porous electrode, homogeneous distribution of metal nanoparticles along the porous layer is normally difficult to achieve. Recently, we succeeded in generating a uniform deposition of Pt within a nanoporous silicon layer by tuning the hydration properties of porous electrodes as well as Pt complex anions [7][8][9][10].…”