Assembly and electrochemical characterization of nanometer-scale electrode|solid electrolyte interfaces.

A technique is herein described for the assembly and characterization of nanometer-scale metal electrode|solid electrolyte interfaces of variable dimensions. The specific system examined in this work involves a sharp Pt tip attached to the piezo-driven head of a scanning tunneling microscope (STM) allowing the tip to be inserted into (or retrieved from) a Nafion membrane placed normal to the direction of tip travel. The actual Pt|Nafion area of contact was determined by coulometric analysis of the characteristic voltammetric features of Pt, using the tip as the working electrode and a much larger Pt gauze attached to the other side of the Nafion as a counter-reference electrode, yielding for some of the interfaces examined values equivalent to as low as 35 000 Pt surface atoms. This rather versatile arrangement allows experiments to be performed in both inert (Ar) and reactive atmospheres, such as oxygen or hydrogen on either or both sides of the membrane, under controlled humidity conditions, and thus sheds light into such phenomena as changes in the overall faradaic currents induced by plastic deformations of the Nafion as well as fundamental aspects of mass transport at reactant gas|Pt|Nafion three-boundary interfaces of relevance to polymer electrolyte fuel cells (PEFCs).

Determination of O[H] and CO coverage and adsorption sites on PtRu electrodes in an operating PEM fuel cell.

A special in situ PEM fuel cell has been developed to allow X-ray absorption measurements during real fuel cell operation. Variations in both the coverage of O[H] (O[H] indicates O and/or OH) and CO (applying a novel Deltamu(L3) = mu(L3)(V) - mu(L3)(ref) difference technique), as well as in the geometric (EXAFS) and electronic (atomic XAFS) structure of the anode catalyst, are monitored as a function of the current. In hydrogen, the N(Pt)(-)(Ru) coordination number increases much slower than the N(Pt)(-)(Pt) with increasing current, indicating a more reluctant reduction of the surface Pt atoms near the hydrous Ru oxide islands. In methanol, both O[H] and CO adsorption are separately visible with the Deltamu technique and reveal a drop in CO and an increase in OH coverage in the range of 65-90 mA/cm(2). With increasing OH coverage, the Pt-O coordination number and the AXAFS intensity increase. The data allow the direct observation of the preignition and ignition regions for OH formation and CO oxidation, during the methanol fuel cell operation. It can be concluded that both a bifunctional mechanism and an electronic ligand effect are active in CO oxidation from a PtRu surface in a PEM fuel cell.