(Keynote) Electrode-Assisted Electroless Metal Deposition – Palladium Nanocatalysts for Organic Fuel Oxidation
Authors: Nakova Aneliya , Ilieva Maria , Boijadjieva-Scherzer Tzvetanka , Tsakova Vessela 
Published: May 2020

Electroless metal deposition can be completed by using a reactive surface consisting of a redox active material that acts as electron donating medium and drives the metal ions reduction reaction. The important peculiarity of the electroless process driven by surface-immobilized redox active species is that the reductant capacity is limited by the available surface-confined redox sites. Moreover, re-reducing the oxidized reactive surface provides the opportunity to repeat the electroless metal reduction without dissolving the once deposited metal phase. The necessary condition for driving this type of process is to keep the initial oxidation state of the reactive electrode surface at potentials negative enough with respect to the equilibrium potential of the depositing metal ions.

Conducting polymer (CP) materials are characterized by several intrinsic interconvertible oxidation states and may easily take the role of reducing agent for electroless metal deposition both as immobilized surface layer on a carrying substrate or as solute species in bulk solution. If deposited as a thin layer on an electrode surface they present the opportunity to perform repeatedly redox-active surface- driven electroless metal deposition.

Electroless deposition of Pd in the absence of solute reducing species is studied on pre-reduced poly(3,4-ethylenedioxythiophene) (PEDOT)-coated graphite electrodes. PEDOT coatings doped with either dodecyl sulfate (SDS) or polysterene sulfonate (PSS) ions are used in the studies. PEDOT-supported Pd catalysts with high-density homogeneous distribution of Pd nanoparticles (NPs) with sizes ranging between 4 and 12 nm are obtained. The amount of deposited metal depends significantly on the pre-reduction potential of PEDOT whereas the observed surface density of the metal NPs is largely influenced by the doping ions used to obtain the PEDOT material. Densely packed overlapping NPs are observed on PEDOT/SDS whereas individual, non-overlapping NPs with much smaller surface density are found on PEDOT/PSS [1-2].

Once high negative potentials are used for PEDOT reduction large amounts of Pd become deposited that significantly exceed the quantity expected to be reduced at the expense of PEDOT re-oxidation alone [3]. Pd particles are observed to deposit also at bare pre-reduced graphite electrode. It is suggested that molecular hydrogen entrapped during the reductive treatment plays the role of additional reductant for the electroless Pd deposition.

Furthermore, based on SEM, UV-vis and Raman investigations it is found that irreversible structural changes occur in the course of strong reductive treatment of PEDOT [3]. Especially in the case of SDS doping this results in a compact and ordered structure that allows for metal deposition preferentially at the polymer/solution interface. On the other hand Pd NPs seem to be present also inside the PEDOT/PSS layers.

Oxidation of glycerol [4] and formic acid is studied in alkaline solutions under voltammetric and chronoamperometric conditions by varying the type of doping ions (PSS or SDS) used in the PEDOT synthesis. It is found that at constant Pd loading PEDOT/PSS-supported catalysts have three times higher peak currents for both glycerol and formic acid oxidation. This effect is ascribed to the lack of overlap of the individual Pd NPs in the case of PEDOT/PSS in contrast to the strong clustering of the Pd observed on the PEDOT/SDS surface. The mass activity of Pd/PEDOT/PSS for both investigated reactions competes with the best performing single metal Pd catalysts.

Acknowledgments: Financial support of CEST Kompetenzzentrum für elektrochemische Oberflächentechnologie GmbH, Wiener Neustadt, Austria (PhD grant for A.N.) is gratefully acknowledged.

[1] A. Nakova, M. Ilieva, Tz. Boiadjieva-Scherzer, V. Tsakova, Electrochim. Acta, 253 (2017) 128-133.

[2] A. Nakova, M. Ilieva, Tz. Boiadjieva-Scherzer, V. Tsakova, J. Solid State Electrochem., 22 (2018) 1901–1908.

[3] A. Nakova, E.M. Anghel, C. Lete, S. Lupu, Tz. Boijadjieva-Scherzer, V. Tsakova, Synth. Met., 247 (2019) 18-25.

[4] A. Nakova, M. Ilieva, Tz. Boiadjieva-Scherzer, V. Tsakova, Electrochim. Acta, 306 (2019) 643-650.