Reports: DNI1049959-DNI10: Design of Pt-Free Complex Metal Oxide Electrocatalysts for Oxygen Reduction Reactions in Fuel Cells

Peter Khalifah, PhD, State University of New York at Stony Brook

Aim 1: Explore conductive pyrochlores 

We have migrated from exploring pyrochlore compounds with highly reduced early 3d transition metals into exploring other structures with corner-sharing octahedra which contain early 3d transition metals.  We produced the reduced titanate compounds La9Ti7O27, La5Ti4O15, and La5Ti5O17, and are investigating there electrocatalytic properties, which were found to be poor.  These compounds are currently being tested as catalyst supports. A paper on the synthesis and physical properties of La9Ti7O27 and La5Ti4O15 is being prepared.

We were recently successful in obtaining funding for and purchasing a sputter deposition capable of producing these materials in the form of thin polycrystalline or epitaxial films which are very well suited for quantitative electrochemical characterization.  This custom system (which has been designed to be capable of producing oxynitrides) has just been delivered.

Aim 2: Explore hollandite compounds

No addition work on these compounds was carried out in the past year.

Aim 3: Produce large single crystals

The focus this year has been on electrochemical testing rather than on crystal growth, so we do not have substantial new progress to report in this area.

New directions – transition metal oxynitride electrocatalysts.

We have additionally followed up on some recent literature reports of ORR activity in transition metal nitride materials such as rock salt type Mo2N, and have been exploring some second generation oxynitride compounds to see if the activity and/or stability of these materials can be improved by the incorporation of oxygen into the crystal structure.  The initial results have been very promising.  We have demonstrated very good electrocatalysis in Co-Mo-O-N rock salt compounds.  In basic conditions (0.1M KOH), an onset potential of 0.918 V vs. RHE (only about 0.1 V lower than that of Pt/C measured under similar conditions) has been achieved, and stable electrocatalysis in acidic solutions (0.1M HClO4)  has also been demonstrated with an onset potential of 0.65 V.  In both cases, a primarily 4-electron mechanism is followed.  The complex structure and morphology of these semiconductors has been elucidated through SEM/EDX, TEM/EELS, XANES, EXAFS, XPS, and XRD experiments.  A set of 3nd-generation oxynitride compounds have been recently synthesized and initial tests show substantial improvements in their electrocatalytic activity under acidic conditions.