Enhanced­ Adhesion of Nanoporous Metal Layers on Modified Glassy Carbon Surfaces

The development of efficient catalysts, mostly involving Pt is very important in modern fuel cells. Pt is highly efficient at catalyzing fuel cell reactions as oxygen reduction and organic fuel oxidation.  The high cost of Pt calls for limited use of the material. To this end, many approaches use nanoparticles. However, capping agents and material loss reduce efficiency of such nanoparticles. It has been suggested that electrochemical deposition of thin film catalytic materials can greatly increase the efficiency of fabrication, while also introducing a new level of control to the amount of deposited active catalyst. Recently, a successful development of protocol for fabrication of NPG has been established based electrodeposition of an ultrathin, continuous film of single-phase alloy followed by a selective electrochemical dissolution (de-alloying) intended for the less-noble component removal. If the deposited alloy is Au_(1-x)Ag_x the newly introduced protocol serve for the synthesis of NPG of thickness that could be as low as 20 nm.

Platinized NPG thin films fabricated on Au substrate have proven to be an efficient and yet low Pt-loading catalyst. However, when processed on low-cost substrates like glassy carbon (GC); the electrodeposited catalyst precursor grows in large isolated and poorly adhering clusters that are associated with considerable material loss upon long-term catalytic performance tests. Some oxidative avenues like thermal treatment, electrochemical treatment and plasma treatment have been explored for improving the adhesion of metals to GC surfaces.  These are associated with opening of pores and substantial improvement of the wettability of GC surfaces. In particular, the thermal and electrochemical oxidation approaches are key treatment options for adhesion improvement. These treatments can be precisely monitored vis-à-vis oxide buildup and respectively oxide layer thickness rendering the treatment process readily controllable.

In view of our group’s long-term interest in the development of nanoporous metal layers on C-based substrates (like GC) for ultimate support of high-efficiency catalysts, the presented research will assess the applicability of oxidative routines in the NPG supported catalyst production. An in-depth look will be taken at continuity and adhesion of the NPG thin films fabricated on thermally or electrochemically oxidized GC surface. The research will primarily focus on (i) the charge accumulation with time during the oxidation of GC surface, vis-à-vis the thickness of the resulting oxide layer. Also, a point of interest will be the metal/alloy deposition efficiency as a function of the oxide layer thickness; (ii) the resultant morphology and the continuity of NPG thin films on GC substrate; (iii) comparison of NPG thin films on accordingly oxidized GC surfaces (i) as a function on the oxidation routine and (ii) to the untreated GC in terms of layer quality and adhesion; (iv) the specific composition of the oxidized layers depending upon the employed oxidation routine.

More specifically, the growth of the oxidized layer on GC will be monitored by chronoamperometry in the course of electrochemical oxidation. The fabrication of NPG thin film on oxidized layers will be followed with a special attention on the deposition efficiency in stripping configuration. The resultant morphology and continuity of the fabricated catalyst support will be derived by SEM as shown by preliminary results in the attached Figure where NPG is fabricated on untreated (left) and oxidized 1 μm in depth (right) GC electrode. The adhesion of the NPG thin films on GC substrate will be tested by mechanical agitation via Rotating Disk Electrode. The comparison of the electrochemically oxidized GC and untreated GC will be measured by Pb-UPD, and (independently) by stripping experiments before and after rotation. The comparative results will be analyzed in detail and critically discussed.

Figure